Because they would have a huge competitive disadvantage in recruiting. FBS schools are limited to 85 football scholarships at any one time. The rules state that a student receiving partial scholarship counts fully against the total of 85. Therefore, if every other program is offering 85 full scholarships and you're offering 75 full and 10 partial scholarships, you're not going to get as good of players. No highly touted recruit would accept a partial if they can go to another school and get a full. Now lower tier sports like baseball, swimming, etc. do give some partial scholarships....but for football, it would only be FCS, Div II, and Div III schools doing this.