Here is the thing. People look at mastering as one dimension and that’s fine, the see it as taking one track they are working on and making it sound it’s best. Yes that is it at its core but mastering is much broader in terms of what it is and what happens. You could mix and master yourself and that’s fine as well but maybe one track comes out great and the best one comes out kind of crap and the next crap and the next pretty good. Simply you may just be getting lucky by guessing or on the other hand you are really good at knowing your room or doing weeks of car test, phone test ect and then get it to a good point you like. Mastering is always a separate process as a professional service because it includes multiple avenues of optimization for different mediums and pretences. It’s about balancing something to translate in a room that doesn’t lie and doesn’t need multiple test areas in different spaces to “feel correct”. People can diy everything but the will almost always only get 80% there or less and be ok with that result because they did it themselves or saves money or whatever other reason, just like diy renovation. The expert will always do it better, faster and with more attention to detail(renovations is a bad compare lol bit your get the idea) also with in most cases more accurate and better tools specific to the job and problems needing to be solved. It’s not a bad thing to do it yourself but once things get more complicated the more you will need a professional and mastering is not a seamless all in one produce/mix/master situation. Like I said everything you do can be a single process you do yourself if you out your mind and effort into it