It's non-trivial to unlock, root, and install a custom ROM - unless you are technical, very few people are going to be able to do this.
Also, it's been several years since I used a custom ROM, but I thought rooted devices were not permitted to use the official Google Play Store? (happy to be corrected if I've got this point wrong!)
Rooting and installing a custom ROM are two independent things, you don't need one to have the other.
Rooted devices can use the official Play Store yes, though most people use Aurora Store to access the Play Store because they tend to remove Google Play Services from their phones as well.
Also, it's been several years since I used a custom ROM, but I thought rooted devices were not permitted to use the official Google Play Store? (happy to be corrected if I've got this point wrong!)