When you are in a university, the grants are only for a limit period of time with a goal (usually 4-5 years) and then you are cut off. The money you get is usually towards tution, not living expenses. Kollel is open ended. They also throw you out of college if you are not able to keep up. Not everyone who starts will finish.
How often do you hear about a Rosh Yeshiva throwing out someone who just isnt cut out to learn all day (I am sure it happens, but its not frequent)
As for as research grants, There isnt just a handout, you have to submit a proposal for the grant and defend why you should get it, It can takes months or years to get them , they are not handing them out so easily. And not everyone who applies gets them, There are alot more applicants than money avaiable