Mostly due to market forces. Not because it takes 5 years to fully judge them.
Semantics.
The primary market forces are that both the coach and school have incentives for not making a judgement for five years.
The coach wants five years minimum before he's judged when taking over a disaster and needs to rebuild:
Year 1: Can't expect much, I'm playing with the former coach's recruits. Can't judge me.
Year 2: My first recruiting class are all frosh--and they don't play much. Most of my rotation is still the old coach's recruits
Year 3: All my recruits still underclassmen--and half are only frosh.
year 4: I should be delivering results by now, but you gotta give me through next year, when the whole team is mine.
Year 5: First year all players are my responsibility.
Consider the case of Kevin O'Neill as a perfect example as to why coaches want five year contracts:
Year 1: 16-13--only 2 more wins, 1 fewer loss than Dukiet's final season.
Year 2: 11-18--nearly as bad as Dukiet's worst season (10-18). Would he really be comfortable being judged at this point?
Year 3: 16-13 - no post season. Three years in, and his overall winning percentage is only marginally better than the despised Dukiet at .482 vs. .458 Despite SIGNIFICANTLY better recruiting, results are about what Dukiet gave us. Lucky for him nobody judged him after just three years.
And schools have incentives from the opposite direction. Five year contracts give them at least some protection from situations where the new coach takes over a strong program and does well in the first few years: Consider Mike Deane
Year 1: 21-12
Year 2: 28-8
Year 3: 22-9
Year 4: 20-11 Wow, that cumulative .695 winning percentage suggests he's the real deal after four years.
Year 5: 14-15 Hmm. Finally playing with his own players and recruiting doesn't look so hot either. Different story
So five year timeframe IS the market force that drives those initial contracts.