For the virus to be spreadable it has to create symptoms.

With Covid for example you would have to make a virus that can compete and attach better to the ACE2 receptor. If it is better at attaching its more bio-competitive against normal use and more likely to make you sick. The key thing with covid is to prevent a cytokine storm, but the 'ideal' virus would still make you quite sick and some would die in the process.

Maybe viruses for bacteria. But viruses to compete against other viruses and win is a recipe for disaster.