I believe what you're missing is the probability that at least 1 of the starting OFers gets injured each year, forcing the 4th OFer into starting duties.
There may be years when the starting OFers stay relatively healthy and the 4th OFer just plays when they need a day off, or maybe as a semi-platoon player. 4th OFer could also slide in if one of the starting OFers DH's for a day, etc.
But there will also be years when 2 starting OFers suffer major injuries and the 4th OFer is going to start everyday for an extended period.
I believe this is why they say that on average, the 4th OFer gets 450-500 PAs a year. He's essentially a "starter". I believe it's similar for your utility infielder.
That said, if teams are using a lot of platoons and such, that may all change a bit - but it does highlight the importance of the 4th OFer and utility IFer.