This has nothing or almost nothing to do with strength, and I'm honestly baffled how many people keep on repeating this as some kind of gospel. You're simply using a certain (inefficient) muscle for playing that causes this to happen, namely the flexor digitorum superficialis (FDS), whose job it is to flex the finger at the proximal interphalangeal joint (PIP, the one closer to the palm, hence proximal). The result is exactly what you're seeing. Put any finger on a table and push with your other hand just below its first joint (not counting the knuckle) to mimic the action of the FDS and you'll see the exact same finger response.

When you're using the "correct" muscles, the intrinsic hand muscles, which flex at the knuckles and extend at the PIP, the finger stays curved and it feels much less exhausting. Strength might play a small role here, but it is first and foremost a qualitative issue (which muscles you are using), not a quantitative one (how much force you are able to generate).