Well said. I think to build on that it should be pointed out that most pro mastering engineers - not mixing engineers who are semi-handy at mastering, but actual mastering engineers - are doing a lot more for you than just making the mix louder. Because, as has been pointed out, simply cranking up a brick wall limiter on the master output is not all that mastering is about. Real mastering also includes attention to EQ, taming peaks and buffing up missing tones and enhancing harmonics and low and high end. This requires an understanding of the psychoacoustic effect - the way our ears are tuned to different frequencies at different volume levels. Also, your mastering engineer will attend to stereo imaging and apply various adjustments that might help open up or balance out the soundstage in your mix. And, the mastering engineer will adjust any intro/outro fades to be appropriate at high volume. And finally, there's more to the volume level than just "loudness". There's also dynamic range, which is specific to different genres and even different styles of sound within a particular genre. It IS rocket science in a way. And just like rocket science it can be learned through years of hard work.
Or you can just turn up the volume and call it good enough. Which perhaps it is. I can't say since I haven't heard your tracks.
Details on the steps our studio takes in its mastering process at - https://www.crazymastering.com/mastering-explanation.html