Sure you could, and there are plenty of tools available for “DIY mastering”. At the end of the day it’s the difference between doing anything for yourself versus hiring a professional. We can change our lightbulbs, but we wouldn’t re-wire the house.
A professional mastering engineer has worked with audio every day, for years. They have high-performance gear, acoustically treated studios, and (most importantly): exceptional ears. They know the tricks to get your music sounding louder, clearer, and punchier, than what someone with less experience might achieve.
We believe it comes down to where you envision your project. Simply put, if you’re looking to get attention on a professional level (radio play, licensing or sync placements, label attention, media coverage), we’d suggest giving them a professionally mastered sound.