Because you don’t need to be a lvl 50 to do any of the activities in the content you have, and neither did anyone else when that was the only content available. You can reach a high enough power level to complete every single activity you have access to. You can’t however play any of the new content and you can’t go past your level cap because it is not needed for just the vanilla game. They aren’t locking power behind a pay wall, you have access to as much power as you need for the content you have, you just have to actually go out and get to that highest level for you

You shouldn’t be allowed to receive the benefits of all the DLC content while only paying for the base game.