Not only can internet connections & buffering be an issue, but particularly on the newer pro-exclusive services (many of which have already come and gone faster than you would believe, leaving the those who relied on them to lose their entire libraries), having songs properly labeled can be difficult. And there's no time during a live event to properly check an unfamiliar song.
So, it's too possible to accidentally load in an explicit song that wasn't properly tagged.
Additionally, streaming services have a risk factor of songs being removed from that service at a moment's notice, for any number of reasons. Artists have been known to pull their music from certain platforms due to legal disagreements.
As a professional sound technician, a big part of my job is to deliver a level of quality that you can't find just anywhere. And I would not be doing my job, if I risked the success of your party by playing my music from a streaming service.
When you hire me to perform your event, you can be assured that I bring my own legally-obtained & owned copies of songs, and that my entire library is backed up more times than you would probably care to know.