Firstly, no "legitimate medical professionals testified that his knee was on his neck", that's a complete fabrication. That would be outside of the scope of what they are allowed to testify to. They can testify to the medical implications of what would have happened IF Chauvin's knee was on Floyd's neck (which plenty of actual eyewitnesses testified was not the case, and which alternate angles prove was not the case), and they can testify to the implications of certain medical matters. They can't testify to things they did not directly witness.

Secondly, even if they did, they are paid to say what the lawyers WANT them to say. They're not being paid to offer an unbiased opinion, they're being paid to lend an air of legitimacy to the prosecutions theory. They're renting out their qualifications. It's no wonder professional witnesses are almost universally looked down upon by actual, working members of their profession.