I'm still waiting for spell critical chance to become a thing, and more spell vamp items since they refuse to nerf lifesteal as they did spell vamp
Spell vamps major nerfs were that it because a unique passive so Vladimir and Jax stopped running around with 4 Gunblades, and it dealt reduced damage in an AoE - because full-clearing a 3,000 HP minion wave with one-skill is not something that should full-heal you.
Life steal in an AoE wasn't a thing when that happened. Unless you got Tiamat, which was awful.
As for stacking life steal - The Bloodthirster used to offer 25%, Zeke's Herald used to be awesome, Wriggle's Lantern used to offer life steal and be awesome, Kingblade used to offer more lifesteal, and Ex Calling used to offer life steal and be awesome.
Also, Bloodthirster's life steal didn't used to be unique.
Bloodthirster's passive is now unique, and grabbing the top 3 life-steal items involves getting Gunblade.
They say it's because of Vlad and how powerful he'd become, but again... all champions that can auto attack can stack lifesteal beyond 100%... so... it's not a valid excuse tbh, and there are counters to lifesteal/spell vamp anyways so not sure why mages keep getting the boot
Getting more than 30% life steal is a challenge within the current itemization, and doing so is exactly the same kind of build-killing idea as going for a WotA Gunblade Rageblade combo on your typical caster - you're missing out on big multipliers in IE, Statik, and Last Whisper just like your missing out on big multipliers like Void Staff, Rabadon's, Lich Bane, and DFG for AP.
Also - to benefit from life-steal you need to constantly auto-attack targets, which you can't do when stunned, suppressed, knocked-up, knocked-back, or blinded.
Now look at champions like Annie, Cassiopiea, Swain, Fiddlesticks, Kennen, Karthus, Amumu, Singed, Brand - with most casters, you deal plenty of damage while incapacitated.
That damage is also front-loaded, so if you incapacitate a caster and then they QWER you, that's their 6~10 seconds of damage all circumventing that CC mattering.
Where on an auto-attacker, stunning them for 1 second is 1 less second they get to life-steal with. Even if you look at the fastest-cast skills, it's increadibly hard to impact spell vamp damage with any CC unless you're targeting a Skarner or Evelynn.
Right clicking has got to get boring at some point.
Implying smashing your face on QWER is any harder.
Simplicity doesn't mean easy, if that were the case Packman and Donkey Kong would not be difficult games. With sustained damage - you have to constantly maintain solid positioning and keep track of everything going on for that 6 seconds of your payload, where a burst-caster like Darius only needs to land 1 skill and mash.
Another thing that doesn't make cents, is how AD champions can deal rival burst damage to mages with unique utilities, with as much range, and with less of a cooldown.
Mages tend to have 1100~1500 range on a lot of their damage.
Marksman are limited to ~650 range more often than not with anything that can benefit from life steal.
Also - most mages deal notably more damage, in less time, and to multiple targets than marksman.
It's not exactly difficult to get 2,000 damage from your burst rotation as a typical caster at level 18, being capable of unleashing it all in 2 seconds or less - when most marksman need ~6+ seconds to match that. Additionally, marksman, unless they purchase Hurricane, are all single - target, where mages are AoE. It's not uncommon to hit 3 opponents with the bulk of a caster's combo.