It started with making office computers personable for home use in Windows 95 e.g. “My Computer”, “My Documents”.

This carried on into web services like My Yahoo, or terms like My Account.

Smashing it into one word was also a thing back in the late 90s / early 00s because it (a) was easy for searching in older search engines and (b) sounded like Apple’s iPod, iMac etc (MyPod, MyMac).

Continued use today is usually because of either (a) it’s been called that for a couple of decades already or (b) the product manager is themselves old and has forgotten how old the trend of trying to make those new fangled computerybobs sound welcoming and friendly.