Because people don't want to ruin their mains trying something new, trying new builds, etc. Also, many people think their mains are already ruined in MMR and they want a reset to demonstrate what they've learned. If you've for example played 50 games of Kench JG with a 42% winrate on your main, that account is totally ruined. Resetting the MMR for it to where it once was takes longer than creating a new account and your stats would probably be bad anyway.
If I could just hard reset my MMR and delete ALL stats on the account but have it start match making at the ELO level I'm at (with the MMR being flexible again), I would never smurf. What happens is that if I play on my main, after a while, the MMR gets "stuck" somewhere which is where I belong at that time with the champs I am playing. However, even if I decide to focus much harder, play better champs, it'll take probably over 50 games to make my MMR good again. I make a smurf, every game feels REALLY important. It has a massive impact on MMR. The first 25 games really matter and I also don't have bad stats on the account for things I've learned I don't want to play anymore, builds I've given up on etc. Legitimately, give me a RESET option for the account and I'd NEVER smurf but all the stat sites would have to remove everything too. For example, let's say my MMR is g2 on my main. Let me hit the reset button, delete ALL stats on it for literally everything but mastery and make my MMR FRESH just like on a new account but start me at G2 level like I used to be. However, make every game really matter for MMR, again, as if I'm a new account in the 7th game of placements after 6 straight wins (which is prob G2 level players). I would have ZERO reason to smurf at that point. I'd have a clean slate and I'd also not be unfairly playing in Silver etc. since it would start me at my main's MMR. This is all I want and my only reason to smurf. A completely clean slate and fresh MMR to give the feeling that every game really matters and you're not stuck. When I smurf, I gain +30 and lose -11. I can be playing with Plat 1 after 25 games of intense focus. On my main, after 25 games of equal quality, I'd still have the shit stats from when I experimented and I'd also be probably just gaining +21 and losing -18 or something like that. Plus, it wouldn't raise the quality of my opponents all that much. Early on for a new account, if you're on a hot streak, you can play at a MUCH higher level very quickly.
Gist: give me a way to get the MMR to be as malleable as a new account, starting from my main's rank (treat me as a new player for that) + let me delete every single stat tracked other than mastery and I'd never smurf assuming the cost wasn't as much as buying a fresh account. If it's for example $25, that's reasonable. Maybe limit this hard reset to only 3x per season.
I've had new accounts that I tried some off meta shit which ended up in Silver 2 or so after 50 games of attempting that shit. These accounts are as good as worthless now and it's sad. If I ever want to play quality players, I'd have to play another 50+ games of my best champs and with high focus and for what? Most of them would be vs low ELO and would be stomps. The MMR takes forever to reset. Moreover, I'd still have those shit stats from the off meta stuff and I don't want that blight on an account I care about. I want to learn and progress and I want to feel that the stats and performance displayed are reflective of my ability or I'd want the stats hidden from view. If I played Talon JG for 30 games and fed my ass off for the first 20, I'd rather not have that displayed. I literally don't show my league account to anyone and no one even knows what it is but it still irks me to have garbage stats that if I were to fix, I'd have to play another 50+ games on each of those champs. I'd rather have a new account which would be more representative of what I learned.
BTW: Someone who is Plat on some champs can legit be high Silver on champs which they don't play well or are really bad in the meta. If it can happen to me, It can happen to others. I am plat for half a dozen champs and the rest aren't to that level. Also, when you get a bad streak to end up in silver, you just don't care about the games as much. You're not as focused. You don't try as hard. You realize the players make ridiculous mistakes and don't listen to calls etc. that it feels wasted to really do your absolute best. On accounts that ended silver after 20 games or so, I basically just take it really chill and don't care about outcome anymore since they're already worthless to me. I'll go get something to drink and occassionally afk under turret till 1:30. I will experiment more and try new champs and so on. If I were playing vs Plats, I'd be super focused and leave base at 0:15 to protect our jg and get deep vision. I'd play mind games with my opponent a bit early to test reflexes and dodging patterns. I'd check their runes ahead of the game. I'd look at the others' runes. I'd evaluate the comp in champ select and dodge if someone's first timing/on a massive loss streak/off role and really bad etc. It's just a totally different environment and mind set. Silver games just feel so random and the MMR is so shit for me to fix that it's not worth playing to my max ability. Thus, I am no longer a plat player when I end up in Silver. I am performing at probably just a low gold level at best and worse if on stuff I haven't practiced. I'm not ruining games as a smurf because my playing ability isn't where it would be if I were on an account I was very serious about.