why is it seemingly so difficult to isolate viruses? various internet schizos claim that viruses like measles, HIV, and even sars-cov-2 have never been isolated in a lab. (caveat: subject to the below definition of isolation.)

it doesn't seem to me like it should be so hard. get sample of people. get some fluid that you think the virus is in. run the fluid through a centrifuge. find the band that the virus is in based upon its density. verify that what you think is the virus is similar across your sample population.

what's so hard about this and why don't scientists do it? or do they?