No one has to take the tests. It is not a compulsory thing to enter the draft.
There were some potential 1st rounders missing (e.g. Douby, and most of the european prospects). Bargnani, for example, was still playing in the Italian playoffs when the tests took place.
These tests can tell a lot about the prospects, but here is a good article explaining why you shouldn't pay too much attention to them:
A couple of extracts from the article:
- The top athlete of the 2003 draft wasn't Lebron or Wade, it was Troy Bell. Brandon Hunter was second, while Chris Bosh was 51st.
-Also in 2003 tests, Chris Kaman was measured to have better foot speed than T.J. Ford
-In 2004 the top athlete was Kirk Snyder. Iguodala recorded the same vertical jump as Redick this year.
- Last year GS's Monta Ellis was ranked the worst athlete, coming out slow, weak and little leaping ability.
-Also last year, Chris Paul was slower than Wayne Simien and Sean May.
So do not make too many conclusions based on the results.