Coming off of 5 years experience playing a Priest in WoW and now the 2 months I've had on this commando (about 1 month at 50 now)...you get tired of healing. Especially with ToR's archaic, option-less, ****astic UI for healing. Holy hell...
When asking why healing/tank classes queue as DPS instead, it mostly comes down to the fact that they're tired of it. Do you know how much easier playing an MMO as a DPS class is for PvE content? You know how awesome it is when mobs die in 3-5 skill casts as opposed to 5-10 as a healer/tank? Especially with the companion system, you know how nice it is to not have all the mobs on you while you're trying to both heal you and your companion AND defeat the enemy mobs?
But moving on from solo content, you know how awesome it is to not have to care if everyone in the group is dying and instead just worry about your own positioning/rotation/interrupts? As a tank and healer, if anyone messes up, you have to cover for them, whether it be taunting that extra pack that got pulled or healing dps who always seem to be slow to move out of fire. Resource management as a healer is always more of a pain than dps. If a dps runs out of ammo, it's bad...but they can still hammer shot while waiting for CDs or whatnot, but if I run out of ammo while healing what happens? The tank dies. We wipe. People complain. But do you want to know why I ran out of ammo? Because I was trying to heal your undergeared/slow/ignorant self. Call me elitist, call me a hater, call me poisonous to the game, but it gets to the point where you just get tired and frustrated and fed up with everyone trying to blame someone besides themselves. Yes, there have been times where group wipes have been my fault since my CDs weren't up and I didn't say anything, or I was too busy scavenging and fell too far behind. But more often than not it ends up being some DPS who thinks they've got god-mode on and charges in balls to the wall and then complains when they get 2-shot.
Do I think that WoW's system of giving extra rewards to what was most needed in queue was a perfect system? By all means, no. But it may have been a step in the right direction. Inherently, we will always have this problem simply because of how Ops and WZs work. You always need more dps than tanks/heals and as you level, dps generally is the fastest. Thus, this leads to a surplus. Also, people who main a healer/tank generally have dps alts that they want to play on too, but I find that a fair amount of people who main dps just level more dps alts, thus creating an even bigger surplus.
So there is no easy fix to this. There never will be unless people start magically becoming unselfish, but we're human and I doubt that'll be happening anywhere in the near future. To those of you that say "find a guild," I have, unfortunately it's a kind of small-ish and many of these people have been playing for at least a year and just don't want to run these damned FPs anymore. Even if they do, normally it's just one person and they end up being a dps, so we queue as a healer and dps...doesn't really alleviate the problem, yeah? I haven't really tanked much, so I can't say if it's fun or not, but hearing from what some guildies/friends have said, perhaps that may be one of the underlying reasons that there always seems to be a shortage of tanks.
That ended up being a lot longer than expected. I think I stayed on topic there and I think I said most of what I wanted to say.
EDIT: On the topic of tanks/heals offering their services for a charge. Nothing wrong about it. If they want to charge people for their time they have the freedom to do so. If you don't like that idea, wait for a different tank to either enter LFG or reply to your post in General chat. No one's forcing you to pay them.