If I had to guess, it’s because rolling isn’t a lot of aerobic conditioning. you go hard for 5-7 minutes and then get to rest and repeat 4-10 times. Bjj like that leaves my body and grips dead but doesn’t exactly work the heart and lungs like I’d want it to. It’s honestly a lot of muscle output if you are rolling hard
Also drilling bjj most of the time isn’t going to be ‘cardio’ either. While drilling striking is almost always a bunch of cardio for newer guys (stuff like padwork or partner drills)
Striking is a ton of ‘cardio’ or aerobic conditioning. There is no ‘rest’ when you are on your feet and someone is punching and kicking at you. Not to mention kickboxing classes tend to have a lot of conditioning built in to the class (it’s part of what you pay for), while most bjj schools I have gone to expect you to condition outside of class if you take it seriously.
If you are looking to feel good after class, try doing some conditioning at the end of bjj. You even said it yourself that Muay Thai you are doing conditioning in the class