I am not touching the title of question much, I am providing a possible reason behind the philosophical difference between Vedas and Upanishads.
First of all talking about whether Upanishads are part of Vedas or not, there are three distinct opinions:
Brahmanas including Aranyakas and Upanishads are considered to be part of Vedas. This is according to ...
It would appear so.
There are other references of vedas in Ramayana.
kushiilavau tu dharmaGYau raajaputrau yashasvinau | bhraatarau
svarasampannau dadarsha aashramavaasinau || sa tu medhaavinau
dR^ishhTvaa vedeShu pariniShThitau | vedopabR^ihmaNaarthaaya
taavagraahayata prabhuh || kaavyaM raamaayaNaM kR^itsnaM
siitaayaashcharitaM mahat.h| ...