This is the answer.

Gay/bi men are a very small percentage of the general male population. The percentage that you see on apps is a fraction of that already small percentage. The men you see on the street are representative of the general population. They're a much larger sample of the population, and you'll likely see more attractive men than in the limited pool on Grindr. This is also why you'll find more attractive men on straight Tinder, Bumble, etc. There are more straight/bi men looking for women, thus a larger sample size, and thus a higher likelihood of running into attractive men.

You might think that the percentage of attractive men should be equal between the gay and straight populations, but this is not true. There is no reason to expect that attractiveness should be distributed across the whole population regardless of sexual orientation because the two traits are not related and emerge independent of one another.