Boxing is a hugely popular sport worldwide, with four major and dozens of smaller entities. The broadcasting and streaming rights for boxing matches are therefore not easy to arrange.
There is no one-stop shop where you can watch every match and free live streams of boxing matches are virtually unknown outside of bookmakers, but most fights can be streamed in some form or another.
Boxing is not a monolithic thing, as many sanctioning bodies and promotions work together at various levels. The live streaming picture for this sport can be difficult to grasp because of that complexity.
HBO used to be the primary source for boxing live streams, but their exit from the industry has added even more complexity and confusion.