I'm not convinced the majority of actual smurfs admit they're smurfs. Lots of non-smurfs out there who'll say they're smurfs from <insert high rank here that they think they deserve> whenever they have a good game to feel better about themselves, while many legit smurfs would rather pretend they're just exceptionally talented new players (or just don't care at all whether people recognize them as smurfs or not).
There's various 'good' reasons for smurfs too. Being able to play with new/low elo friends without completely ruining the game for them on your main account (which would have a way higher mmr typically), being able to comfortably explore different champs/roles in a fresh enviornment (people maining one role at a plat level may only play another role at a silver level and not be able to effectively learn at their own elo with constantly getting crushed), being able to 'escape' from your main account if you don't want to deal with various friends or whoever who might be bugging you to play with them. Sometimes people just want to double check the frequent claims that it's impossible to climb from bronze/silver due to trash teams (and usually confirm that it is indeed straightforward to climb if you actually are playing at a higher level).
Some people are doing it just for ego or to avoid big queue times, or for unranked to X in Y time youtube/twitch things. Others are just intentionally trying to delve deep into bronze 5 for the lulz. Theres other reasons too I'm sure, but the point is theres a fairly large variety.