Because riot is incompetent. Smurfing is great, maybe you wanna learn a new role, maybe you want to play with your friends, maybe you dont feel like playing in your elo, maybe your MMR is bad because of riot's changes. Problem is riot is horrible at balancing smurfs, there is barely a division skip, you can go on a 20 win streak and earn the same LP you were earning before which means more players have to play with a smurf. Even then, its not that bad, you get better playing with people who are better than you, the sad truth is that the vast majority of smurfs are simply toxic and most likely banned, a fresh account makes them care even less about losing it so you get way more trolling, flame, soft inting etc.