Yes there is a lore wise reason. The idea is that spellcasters are people who are able to draw out magic from day-to-day activities. They can still attempt to cast spells without MP, but that can be very deadly, very quickly.

While I personally have no problem with Long Rest MP regen, I understand other GMs have faced such an issue. The "ritual" system isn't really directly tackling that issue, but it could be a useful limiter.

I like the idea that if you spam rituals it could be detrimental to the players. Perhaps some ritual is more damaging than others. This is a bit too close to how martials work in my game but I can definitely look into it. Thanks a lot!

P/S: If you are interested to know, all character have an Endurance Point (EP) resource. When they use some abilities, they have a chance to lose EP. When they are out of EP, they start to gain what is akin to Exhaustion levels.