1) Because it apparently aids performance and you can evidently get away with it, see this case as the most recent example.
2) Because it's part of a roid cocktail - typically cheats use more than just one drug for synergistic effects.
FYI, for those we still haven't done so, it might help to read what Professor Johnson wrote about the likelihood of accidental congestion (point 84 of the AIU ruling):
Professor Johnson rejected the theory ... for the following reasons: ....
And what did CAS (i.e. two Canadian and one UK judge) say?
Jarrion Lawson is found to have committed an Anti-Doping Rule Violation (ADRV) for which he bears no fault or negligence
The CAS Panel found it more likely than not that the origin of the prohibited substance was
contaminated beef consumed in a restaurant the day before the test.
"more likely than not": they didn't exactly clear him, but ok.