Multiclassing is still a thing because people associate it with D&D and want it to be a thing. IIRC, there was a fair bit of backlash to 4e for not having "real" D&D style multiclassing.
As for a gameplay reason to not remove multiclassing; characters who are not dedicated spellcasters (Wizard, Sorcerer, Cleric, Druid and Bard) or Paladin scale very, very poorly past lvl 5. So if you want a martial type character that is reasonably strong next to a well played spellcaster, you have to multiclass. BG3 gets around some of this through the way it handles equipment, but it can only go so far when spellcasters also get awesome equipment.
