It's the far right side of school life, where the frat is a hierarchical in-group. The pledges put up with it for access to the priviledges that come with being in the group and ones that go through with it have a mix of a) thinking it's paying their dues now to reap the benefits later, b) don't appreciate how much it might escalate until it does, and then have peer pressure and sunk cost fallacy driving them to continue, and c) the escalations might also excite them because if this is what they have to put up with to get in, if this is acceptable for senior members to subject junior members to, just imagine what the frat will let them get away with when they are full members or senior members. It's like the "it's ok for billionaires to treat my class like shit because I'm planning on joining them one day" mindset, except with an actual path to joining them soon.

There might also be some S&M aspects to it where some on the M side even enjoy it, though I can't really relate to that myself so it's only speculation.