That's actually the perfect question to ask a sports apparatus currently trying to straddle being both a semi-professional sports league, and a college-based sports league with conflicting motives, rules and revenue systems lmao
But also yes, it's 100% arbitrary. They don't kill you if you don't graduate in 4 years.
Edit: On that same token, if it's about bachelor's degree time, why do they allow grad students to play? If you earn a degree in 3 years, should that kill your clock? I don't think it should work like that. In fact, if any truly psychotic football player wants to play while getting a PHD, more power to them lol