The same reason you pay full price for everything you buy, but the store you buy it from pays half of what you foolishly gave them. And don't get me started on taxes...
(Me, I want to know why the ONLY people who will pay full price for Ritual scrolls are the PCs. According to the rules, no one else will pay more than half price.)
(I know the game balance reason -- they don't want PCs becoming scroll merchants. The old way of solving this was via XP costs. With that eliminated, ham-fisted rules that make no economic or worldbuilding sense must be instituted. So it goes. Would be simpler, IMO, to simply remove the ability of PCs to make magic items at all, or make it nightmare inducingly complex and time consuming, as it was in 1e/2e. But that would conflict with the design goal of, basically, letting the players have whatever level-appropriate magic items they wanted.)
Getting back to the original topic... I think it's deliberate that it's not that hard to acquire ritual casting. There's not a whole lot of benefit to the party to have more than one character able to cast rituals, and most parties will have either a wizard or a cleric in them who can handle the party's ritual needs. Making it easy to acquire by feat just means that a party doesn't get screwed over just because nobody felt like playing a cleric or wizard that game.
This is a good point. How exactly is it abusive for a fighter to have access to rituals? They serve no combat function, but any party without a ritual caster is screwed.
Well the sneak attack one is a bit weak too given that you need to be using a light blade or xbow to use it. I'm not sure that any class other then rogue is likely to use that.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.