From what I've read they would fear that the gods would be insulted if they were 'given' less than the best the people had to offer. And ofcourse, being gods they would know what was available.
You know these people felt storms and bad weather were because the gods were displeased with them- so they certainly wouldn't do anything to piss them off. If this is true today, we must have some pissed off gods because we sure have gotten a lot of bad weather lately. Time for some better quality sacrifices?