I found up to 3 T-60s already and I'm not even that deep into the game. For a set of armor that wasn't even officially released it should have been considered as a unique variant not a common spawned item.
You've not met the Brotherhood, since you're not doing any of the main storyline, have you?
All BoS Knights in power armour wear T-60.
It's a bit silly, unless the BoS found the means to create their own Power Armours, and were producing T-60s from pre-war blueprints.
But I also believe it has been said that the creation of power armour is too difficult for post-war people?
Then again, as of Fallout 4, Power Armour isn't anything too complex. Even raiders are fitting makeshift power armours over the frames. And would the BoS not have the technology to produce such stuff, especially if they have complete control of the whole of the Capital Wasteland and all its various factories?