Probably because they’re the only class that can escape anyone they want, even zergs if they want to.
An ele can’t RTL away if he’s rooted, but a thief can infiltrator’s arrow out of roots and can hit random animals to stealth away.
Basically, thieves can reset fights. Yes, thieves can lose to several specs, especially confusion or retal ones, but their difference with other classes is that they can still harass and disengage at will from even fights that do not favor them, whereas a necromancer for example is kittened if a competent thief catches him, or a mesmer is screwed the moment a d/d or engineer arrives.
Unless the thief is really determined and set up their utilities before hand for the express reason its not always that easy to escape, the moment your fighting someone that has learnt stealth isn’t a get away free card it becomes a lot more complicated.
The issue with players believing thief is OP comes down to 2 things, the first is stealth, this is a mechanic that is almost entirely based on the players at both sides, its one mechanical advantage is the anti-target lock system, which stops a select few moves, outside of that it is all about who plans the routes and such better, unfortunately in low tier and wvw this favours the thief, in higher tiers it favours the enemy.
Sure even the best person can’t always get the thief but once you know the general plays a thief can do while stealthed its not hard to prevent either their offense or their escape but it comes down to the players themselves rather than buttons pressed in game to counter or use the stealth well.
The second issue is the misconceptions about initiative, many many people see initiative as an infinite resource that allows endless spamming of moves, while it does allow a thief to utilize a weapon skill more than once initially once it is spent it is then equivilent to a cooldown. Granted at base regeneration rates the cooldowns tend to be 4, 5.32, 6.65 and 8 second cooldowns which is a lot faster than everyone elses which tend towards the 8, 10, 15, 30 pattern, however unlike those cooldowns all thief weapon skills share the same cooldown resource.
So while say a staff necromancer can hit hits 5, 4, 3 and 2 skill back to back and then each will have thier own individual cooldown a d/d thief attempting the same 5, 4, 3, 2 skill use would run out of initiative on his second skill (cnd 6, dancing dagger 4, 10 initiative spent, 2 left and 3 being the cheapest skill), and then as the necromancers skill recharges he can then pop them again and they would not effect the cooldowns of the remaining skill, but if a thief then popped his 3 point skill once he got that missing single initiative point all his weapon skills are now essentially on their full cooldown again.
Add to that the staff necromancer can then press ` and switch to a second weapon set that will have a fresh set of cooldowns to use while the staff ones cooldown while a thief swapping weapons still has no initative.