Because they do.
The reason may be something to do with naming becoming non-unique if you and Jobober cross to a zone with another Jobober in it. Although I would have thought that could be easily addressed in coding. Beastlord pets used to be able to zone with them and apparently this is why they are called Soandso's warder and no a normal pet name. The same may be true of Wizard familiars.
Earth may be fine to solo with but it causes havoc in groups with the random rooting. Each has it's special properties and I'll leave it to a better mage than I am tyo explain them - or throw you a link to the mage sites.
To a certain extent it depends on the mobs but since casting and kitting out a pet consumes a fair bit of mana you can't keep swapping. Air pets stun which is very good for caster mobs.