In the first Captain America movie, a frail Steve Rogers also asked why he was the one being chosen. Yes, I know your question is technically not about the why, but the answer Erskine gave him still stands...
ROGERS: Can I ask a question?
ERSKINE: Just one?
ROGERS: Why me? [...]
ERSKINE: [...] But more important, the man. The serum amplifies everything that is inside, so good becomes great; bad becomes worse. This is why you were chosen. Because the strong man who has known power all his life, may lose respect for that power, but a weak man knows the value of strength, and knows... compassion.
ROGERS: Thanks. I think.
ERSKINE: [Gesturing toward the wine] Get it, get it. Whatever happens tomorrow, you must promise me one thing. That you will stay who you are, not a perfect soldier, but a good man.
Sam Wilson, while not necessarily a "weak man", does qualify for the role - see his "good actions", good heart etc throughout the MCU. As @TheLethalCarrot noted, he's a decent fighter to begin with - and comics-wise, Sam Wilson as Captain America in All-New Captan America was holding his own nicely (with no serum involved, IIRC).
And in case he needs the occasional amp-up, he's friends with gods, superpowered people, the Sorcerer Supreme, Professor Hulk and all eight of his PHDs - he'll be fine.