Because you're bad at dealing with her and let her play the game exactly like she wants to?
Hello, Lux main since two years back here. Lux may have gotten a boost in strength lately but she's not the freakin' terminator here. She works well with thunderlords and her E is easier to land now... she also got a little help from Luden's Echo being introduced which bolstered her otherwise sub-par waveclear and mobility... AND her strongest period in the game is mid game... the point at which most games END now.
That said, she has the same flaws she's always had. She's immobile, squishy, 100% skillshots, and a little mana hungry. It's ironic that she's considered so strong now when for ages she was THE definition of a balanced pick mid. She used to struggle against both hardcore tanks AND assassins, the late of whom countered her in lane pretty hard BUT she could handle by just playing very, very safely. Riot nerfed MR, making all mages stronger overall, then made AP items more expensive, making mages with good base damages more viable early on. Lux feels strong now because most of her rivals are in the dumpster. Xerath used to out-perform her as a poke/seige mage but he doesn't combo to abuse thunderlords as easily so he's ignored. Ziggs used to outperform her as a poke/waveclear mage but he got nerfed for it. She has no rivals right now at what she does.
What's more, AD mids are VERY common now thanks to all the juicy AD items being thrown around, and games with more than one adc are ALSO a thing. I'm looking at you, Graves jungle/mid/top. The role Lux performs best against? Adcs. She outranges even Caitlyn and she has some nasty burst for their squishy butts. The abundance of AD mids also means a lot of MELEE mids, which she can harass brutally without much fear of reprisal. Also, come later on in the game, she will have more waveclear than most AD mids (Once she has Ludens, at least) and they'll suffer in seiges unless someone else on their team has an answer for sieges. Most assassin mids don't. Add in the fact that the teams often go very AD heavy and Lux gives her team an advantage just by providing strong magic damage, forcing them to not stack JUST armor, like her team can against them. The only other popular magic damage mid right now is Ahri... an assassin. Of lesser popularity are LeBlanc and Diana. ALSO assassins. Both having issues with waveclear, too.
Lux is strong right now due to the meta, not her numbers or her kit. She needs other mages to be brought up to her level a little. Until it happens though, here's some advice for beating her.
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Dodge: Obviously, she's all skillshots. Her E is painfully hard to dodge, but it isn't game-breaking on it's own, especially early. DODGE HER Q. Her snare is thin and sets up her entire combo. 90% of the time landing her Q is how she gets kills. Avoid that at all costs. EVERY champion has mobility these days. Use it.
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Roam: If you're an assassin mid and Lux is playing safe... roam. She's hard to kill in lane if she's playing defensively (and most Lux do) but she's also slow, fragile and vulnerable to being overwhelmed if she leaves the safety of her tower. Roam. Go bot and snowball them. Go top and snowball the bruiser. Coordinate with your jungler's movements to 4 man bot or even bait a gank bot and ambush Lux if she tries to follow. Many won't. If she doesn't, just punish other lanes hard and snowball your team. This is the best way to beat a turtling Lux. And don't just try to feed the carry... Lux can burst the carry. Feed that top lane Wukong or Malphite. She struggles against people with gapclosers who are kinda tanky. Lux can't do shit when your 6/1 Malphite ults her under tower except try like hell to run. Get someone sturdy enough to survive dives and Lux's impact on the game is VASTLY diminished.
3: Punish her: Any time she misses that snare, go in on her. If she's hugging tower and you're not tanky perhaps not, but her E pushes lane if she farms with it. She has to push at some point, and once her Q is down she is pretty defenseless. Gap-closers win here. Mid to late game any missed binding should see someone attempt to punish Lux if at all possible. In lane, use it as your opportunity to harass her or even dive for a kill.
4: Tanks: I know, everyone wants to be the carry or assassin and get kills. Lux fares very well against squishy teams, no matter how much damage they pack. What she struggles against? Tanky hard-engage teams full of people like Malphite, Jarvan, Diana, Xin Zhao, Wukong, Amumu, etc etc. Save Wu, not many of these champs are popular now, but they brutally punish Lux's lack of speed and durability. I have lost mid lane to... no joke... a Jarvan mid, who built hydra first froze lane near his tower and just all-ined me with his flag pull every time I came out to even ATTEMPT to farm. Just don't pick tanky people who are easy to kite like Darius unless someone else on your team has the hard engage to start the fight so you can waddle in.
In short, give the meta time to shift and she'll fade back to normal. Her counters aren't popular picks right now and nearly everything is going her way in terms of normal team comps and how people play the game. Either you can DARE to break the meta and play different champs and comps than the ones everyone else is playing... or you can just try to abuse her weaknesses as best you can until the meta shifts and she's not longer in such an ideal spot. Or the old fail-safe way to get better... play a few Lux games and see what people do that gives you a hard time. The easiest way to learn how to beat a champion is to play them and learn their weaknesses.
Try it, and good luck!