Because DPS is easy to play, you just hit your rotation and be done with it. You dont need to babysit people, not have a great tactical understanding of things.
A support has no rotation, you have to act accordingly to the boss abilities, group setup and performance of said. Each fight is different, what worked once may not work the second time and you must adjust your game all the time.
At the same time, if your DPS players are terrible, then you can not carry them at all. Keeping someone alive, does not deal damage, but damage is required to kill things.
I had it so often that nobody died, even if they did the most silly things, but their damage was so poor, it was a nightmare.
And yes, people dont treat support players right. They want them to be weak in pvp, be cannon fodder, not have any DPS and if a support is in group, all of a sudden pots are no longer in use, even flares and other battle items, because that is all the supports job…
If you think there are not enough supports, then just play one. Go find out how life is in MMO´s for that class. 
Needles to say, prices for support gear are super high and yes, I have seen it multiple times that people did roll for the engraving book supports use, as DPS.
At WOW, almost every class can be a support, be it tank or heal. Yet, there is always a shortage in non guild content. Releasing Artist does nothing, people will not go out of their comfort zone which is DPS.