Although other fast food restaurants offer sides like onion rings as an option, you don't see them on a McDonald's menu because they're simply not profitable enough for the chain.
Former McDonald's corporate chef Mike Haracz said in a TikTok video that the chain is most profitable when it can efficiently produce and sell its products in large quantities.
So, it doesn't make good business sense for the restaurant to sell onion rings because they are far less efficient and profitable than french fries, which have a very high markup.
For onion rings to be profitable, customers would have to pay a dollar or more for it than they do for fries. That's why, McDonald's would rather sell you their french fries.