Its because credentialism has permeated the software business recently. Credentialists love tests, its how they got their credentials, and if they don't keep believing in tests over real world performance, they will have to re-examine the true worth of their credentials.
Interviewers are coming to realize credentials, be they Microsoft certs or computer science degrees, are not great indicators of software engineering aptitude.
The more you believe in the accuracy of credentials, the less you would want to ask coding questions in an interview. Better to simply examine resumes for the appropriate certifications and then ask fit questions.
If on the other hand, you believe nothing but on the spot coding demonstrations can prove a candidates abilities, you would do exactly what these interviewers are doing. Perhaps these tests are not the optimal tests, but they sure beat credentialism.
