Smurfing is a direct result of rigged matchmaking systems. The devs created this problem with SBMM. Without SBMM there would be no reason to smurf.

There's a common misconception that a fair matchmaking system (no SBMM) would result in matches where noobs get destroyed by skilled players. That's not true.

Skill distribution in multiplayer games follows normal distribution, which means that the overwhelming majority of the playerbase is around average skill level. The further you are from average, the less players there are around your skill level. With a fair (random) matchmaking system the overwhelming majority of any lobby would be around average skill level, against whom new players would have a fighting chance.

Back before SBMM was a thing multiplayer games were fair and fun for players of every skill level. Think of games like COD4 and MW2, for example. Did you ever hear about noobs complaining about getting destroyed over and over again in those games? I didn't. MW2 was the first multiplayer game I played extensively. I was bad when I started and I got good over time and had fun through it all.

People don't make smurf accounts to bully noobs. They make them because SBMM rigged matches are not fun and they have terrible connection quality and queue times because of how drastically SBMM limits the amount of players you can matchmake with. In Apex Legends, for example, only 0.2% of the playerbase get in Master/Apex Predator ranks. Imagine a matchmaking system that lets you matchmake with 0.2% of the playerbase and leaves 99.8% of the playerbase out of your matchmaking pool.

Nowadays every mode in every multiplayer game has SBMM. Even the so called casual modes.