Well, he did. It's just that Mueller was told to investigate:
(i) any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump; and
(ii) any matters that arose or may arise directly from the investigation; and
(iii) any other matters within the scope of 28 C.F.R. s. 600.4(a).
Rod Rosenstein’s Letter Appointing Mueller Special Counsel - The New York Times
And being a prosecutor, he focused on crimes. "Collusion" does not appear as a crime in the U.S. Code (that's why you focused on the word, of course). But criminal conspiracy does. And the only reason Trump skated is his people managed not to leave any evidence of an explicit agreement.
As the Mueller report explained, and the senate intelligence committee added to, Trump knowingly invited and received Russian interference in the election on his behalf.
Then there were the 14 instances of obstruction of justice Mueller emphatically did
not clear Trump of, but was not allowed to indict Trump over (or, in his view, say that Trump was guilty).
Then there was Trump openly trying to extort announcements of sham investigations into Biden out of Ukraine using unlawful withholding of congressionally approved aid.
Then there was months of lying and denying on COVID, that has gotten over 170,000 Americans killed - nearing if not past three TIMES the number killed in Vietnam.
And those are just some of the highlights, all standing out in a sea of corruption, criminality, and profane incompetence.
But Trumpists don't care about the country. They'll happily destroy democracy itself - whatever is necessary - just so long as they can tell themselves they "owned the libruls". It's tragic, really.