Four organisations, one division
Unlike sports with a single governing body, boxing has several major sanctioning organisations that each maintain their own rankings and crown their own champion in every weight division. They emerged at different points in the sport's history and none has authority over the others, so it is entirely possible, and common, for a division to have four different fighters simultaneously holding a version of the title.
Belts versus genuine standing
Because of this, holding a belt is not the same as being recognised as the best fighter in a division; a fighter without any title can still be considered the true number one by pundits and rivals alike if they have beaten the fighters who matter. Fans learn fairly quickly to treat belts as one input among several, alongside head-to-head results and the broader consensus among people who follow a division closely.
Why unification is hard
Bringing belts together into a single undisputed champion requires the actual belt holders to agree to fight one another, which involves competing promoters, broadcasters and purse negotiations that do not always align. This is why undisputed champions are relatively rare events worth celebrating specifically, rather than an expected outcome of a normal, functioning division.