There are some UHF portables that do give you the full 5 watts, just not all of 'em.
You say "the full 5 watts" as if it's some sort of gold standard. It's not, it's just an arbitrary number that some of the more popular portable radios were capped at because of battery limitations, as Steve pointed out.
Generally speaking, the lower the frequency the higher the power output. However,that is not always the case.
For example, an APX 8000 puts out a maximum of 6.3 watts on VHF, 5.3 watts on UHF, 2.65 watts on 700, and 3.3 watts on 800. In low power mode, it's capable of going as low as 1 watt on all bands.
Don't get too wrapped up in numbers. There is a lot more to the effective power of radios than raw wattage, not the least of which is the antenna system.