Sanskrit and e, a, o

Why do linguists say that Sanskrit only had the (basic) vowels i, a,
u and not the vowels included in the Latin/Greek e, a, o when it is
obvious from the extant earliest Sanskrit texts that they had e and o?

Why do they ignore the existing extensive evidence in favor of some
hypothetical construct when it seems apparent to me that humans
probably had those sounds from near the time they diverged from
chimpanzees.

What is the rationale here? (It had better be good. :-)