Because they want you to play the whole game, not just pvp. Same reason you could conceivably hit the level cap by doing strikes and nightfalls, but it would take forever. They want us engaging everywhere. I think we will eventually get to a system where it does require X power level since I believe the power grind as-is is likely to go away. I do, however, like the power advantage. I won't argue against having more ways to hit it, but I do think it's a good thing to have to reward your players for time investment. Not to mention literally having more power (level) SHOULD translate into having more power if you get what I mean there. TLDR I don't see this as a problem currently, but I think it won't even be a thing after season 21.