Two reasons:
1) Variety for me: a choice of classes, races and current quests/activities to choose from, so I can play what I feel like at the time.
2) All my characters are different people, with different personalities, priorities and ways of doing things and different stories. If they're too similar to my existing characters I wouldn't keep (or create) them. I use them to do different things and tell different stories so it makes the game more interesting for me.
Importantly this means a lot of the problems other people raised don't come up for me. My alts are not clones churning out the same writs, dailies etc every day so I don't need to rush training them all to an identical status to feel they're usable. I have 1 crafting character, 1 who does scrying, 1 in the Dark Brotherhood etc and most of them (including those) focus on questing so I can start playing them right away.
I've been thinking about an arcanist class alt.
Firstly, the best race for arcanists are orcs (stamina, weapon damage, health), despite being quite far from magic knowledge and spells. Also they are ugly.
I started in 2017 and still don't have a single alt.
Sometimes i make a lowbie alt, but very quickly i realize how much of new "work" i made for myself this way, and delete it.