Most TLDs allow registration lengths up to 10 years. And registrars will happily sell you registration for 10 years. Buy the domain, then immediately renew it for 9 years. I suspect the reason they don't offer a choice of registration years in the initial checkout flow is to reduce purchasing friction -- they don't want you to have to make another choice and then potentially back out of the sale. It's the same reason e-commerce sites will ask you for as little information as they can get away with when checking out your cart; for digital sales many retailers just do zip code instead of full address since they're not shipping anything.

As for longer registration periods than that, I suspect it's the same reason you can't, e.g., call up your cable company and try to pre-pay 50 years of service. They have no idea what things will be like that far down the road, and they don't want to have such longstanding obligations weighing on them.

It comes down to business decisions, not technical issues; technically speaking the spec allows for up to a 99 year registration period, though I'm not aware of any TLDs that support that. https://tools.ietf.org/html/rfc5731#section-2.5