Trademarks.

And the patients generally don't have a pronunciation key. Doctors prescribing might not, either.

Drug names are frequently BS loaded with strange consonants without singular pronunciation guidelines (sadly English isn't German) so humans already have a lot of variation in how things are pronounced - models then need to comprehend those variations, regional dialects (like those fuggin' ahs from Boston).

It ends up being quite a lot of variation.

As some examples: Wegovy, Ixempra, Qvar, Keflex

People pronounce drug names all sorts of ways, and there's a lot of them (with a steady trickle of new ones)