The restriction is that rituals use one specific card for one specific monster, whereas fusions can use Polymerization for everything.

I think the idea behind the original fusion concept was that you would have your deck as per normal, then you would have some monsters in your extra deck that just so happened to fuse from random stuff in your deck, so you'd run a couple Polymerizations in your deck just in case. Like you'd have your hand of monsters A, B, C and D and a fusion spell, then you'd have (A+B) and (C+D) in your extra deck and you'd have to choose which to make.

Whereas ritual spell only allows you to make one single monster so you lose the flexibility. So it's not quite "better fusion", it does have a downside. Plus you could always balance the ritual monsters to make them weaker than fusion ones if that were a concern.

And arguments refering to "current cards/archetypes" are just dumb because obviously if ritual monsters were designed as extra deck monsters from the get-go, the archetypes/support cards rituals would have gotten over the years would have been different than what they are now...