A little reasoning for the upgrades:
Domestically:
very few people pay for first class. it's good but not necessary enough for most people to justify the cost. united's goal is to make the most money possible from each plane. no matter how good they make first class domestically, not that many people will pay for it. so they've made it a decent product that gets a lot of upgrades. the upgrades make elites happy and keep elites buying ua tickets and also frees up last minute space for standby and more importantly last minute economy fares (which are expensive=$$$ for united).
Internationally:
at least in the case of BusinessFirst (GlobalFirst is a bit different right now):
people do value this and pay big bucks for this or instruments (ie. miles and global premier upgrades which frees united of previous committments). United makes revenue this way; if they give these away for free no one will buy the seats. Since they make so much money off BF seats, it's more worth UA's while to leave a couple empty than to make a practice of giving them away. Also the last minute economy seats available are not as big of a deal since people jump onto a last minute international flight less often.
GlobalFirst-supposed to be very luxurious, exclusive for very posh customers or instrument supported upgrades. these days not as luxurious, ua management looks to be trying to phase out.