Pi-Hole comes with a basic set of filter lists. The internet is full of more filter lists. It would be a cool option if you were asked during installation what the focus of the filtering should be.
For instance:
The installer would then install and activate carefully compiled filter lists form the community.
Pihole does not maintain any filterlist itself. The current "default lists" have been added to give novice users something to start with. They are known to be well maintained and trustworthy. I think it is outside the scope of pihole and the capacity of the developers to judge and categorize lists from the internet. The more default lists will be added, the higher the risk of false-positives which will led to complains in the forum, why site XYT.com is not accessible. The team would then have to contact the maintainer to remove XYT.com.
I think the two basic lists are a good start and everyone is free to add as many adlists as they like.