They haven't at all , it's just they will now need a visa.
Yet jazz musicians travel the world to various festivals including those in Africa where they also need a visa to work.
Musicians doing world tours are a very limited number, largely because the logistics of doing so are an utter nightmare.
Small UK bands playing at small festivals, or finding their own tour across Europe has become normal. That will almost completely stop now because the logistics of doing so will be an utter nightmare.
School choirs and orchestras regularly visit the EU, either as part of school exchange programmes or as part of civic events. This will also stop, as the logistics are now an utter nightmare.
With all the above you don't just need to worry about the visa issues, there's also the paperwork for the equipment and instruments. Many classical musicians have found they can no longer take their instruments into the EU because they're made from wood and they have no documentation to state that the wood used in the manufacturing process was clear of parasites and other things that can be carried by organic materials. No documentation means if caught it will be disposed of by staff at the border. Not great if you're playing a 100 year old violin worth millions. Orchestras have to have this documentation for every component of every instrument. Then there's the customs documentation, everything has to be recorded and tagged so you don't leave anything behind or bring anything back without paying the appropriate import / export / vat / customs duties on it. Not so bad if you're just a singer, pretty difficult if your touring 3 trucks filled with cables and equipment needed to put your show on.
All of this also applies to visitors coming in from the EU. The days of "we'll fly you in, bring your guitars and we'll provide the rest, stick you in a hotel after and fly you back the next day" will have gone. Those logistics, which were great for promoters and gig-goers as it meant diverse line ups and getting famous acts from Europe for one off shows was trivial, are no more. Once you have to start sorting visas for everyone, carnets, customs documentation and so on, along with official invites to be sent to the authorities and documentation about how the job can't be performed by a local instead (all stuff that is done for non-EU acts at present) it's just easier not to bother. Ever notice how we have a lot more European acts on British stages than we do African, Asian and even American acts? Those that do come tend to be on formal tours to recognisable venues, which behind the scenes will be promoted by the likes of live nation who have the resources to do all the leg work.