Why do people like underdogs so much? I could throw out a half dozen simple, baseless reasons, but I'm not going to do that. Going through basic writing theory, underdog characters are well-liked because their sympathy and active "sliders" are cranked up very far. By contrast, their competency "slider" often starts relatively low. The characters' arc often deals with competency, which helps as well.
When smaller people get beaten up by bigger people, people try to make the best of terrible living conditions, or people try their hardest to accomplish something and fail anyway, they incur sympathy. This is why stories with underdog characters often open with a few scenes or chapters of those characters being put through the wringer in one way or another. In a football movie, for example, the "viewpoint-centered" team typically falls short in the first several games, and may lose one or more key players. Zooming in on the particularly bad situations of certain players can increase the sympathy for the whole team via the 10/90 rule of thumb, whether the other players would organically draw sympathy or not.
Underdogs are usually quite active. They care about succeeding and they work hard in order to have a chance at doing so. As they work toward their goals, they grow and learn, which helps them to become more engaging. Most readers like characters who are so active that they challenge antagonists they have little chance of overwhelming.
At the beginning of stories featuring one or multiple underdogs, said underdogs tend to be low on the competency "slider." If it's a group, some of the characters may be competent, or if it's an individual, they may have some skills that are well-honed, but overall they aren't very competent. This allows them to grow as characters, often in a clear way. If a team starts winning a lot of games when they used to be losing them all, they're obviously becoming more competent. The fact that the main antagonists of underdog stories tend to be more competent than the underdogs they're facing only aids in the underdogs' sympathy "slider." (Just make sure the antagonists aren't more engaging characters overall.)
So, yeah, there are three main reasons underdog characters are loved by many. They are almost always high on two of the three character attribute "sliders" and usually have a decent arc that develops the third.