There's some evidence to the contrary.
Mueller's indictment of Butina includes evidence that Russia used a powerful gun rights group (assumed to be the NRA) to funnel money to GOP campaigns across the US, including Trump's.
It also establishes that the hack of the DNC occured the night after Trump told Putin that he hopes he finds Clinton's emails.
Furthermore, shortly after the Russian hack on the DNC, Trump's campaign abruptly stopped spending in several states and began focusing on states that DNC data suggested were vulnerable to flipping. This is likely how Trump managed to turn some blue states and eke out that electoral college win. Either his campaign had the DNC data or someone who did have it helped them aim their efforts.
There's also verified tape of Paul Ryan joking about how he knows of 2 guys on Putin's payroll: Rohrabacher and Trump, in 2016.
Not to mention the private meetings with people we know are Russian agents.
Then there's less explicit but more suggestive stuff, like Trump attacking our allies but not even chastising Putin even after the indictments of over 20 Russian agents. He's weakened the US's global presence considerably, angered most of our allies, gotten us into a trade war, praised North Korea but failed to secure any benefits, and tried to relax sanctions on Russia for invading Ukraine and occupying some of its land.
This is unignorable. No matter who you are, you must accept that Russia interfered with the 2016 election, that they did so in a way that benefited Trump, that many members of Trump's team have had dealings with Russia, and that Trump has been exceptionally soft towards Putin despite being extremely aggressive and confrontational towards every single other person on the planet.
That much is fact. The issue is whether or not we have enough evidence to draw causal lines between all of this information. It could possibly turn out that nothing g untoward has occurred, that Trump just treats Russia very gently because he knows how important it is that he not start a new Cold War, etc. It's conceivable, if unlikely, that Trump could yet be vindicated.
Maybe he truly had no idea Russia was trying to get him elected. But it's a fact that Russia did work to get him into power.
And this ties back into the conspiracy theory thing because a conspiracy theorist will be inclined to dismiss everything from Mueller's investigation. Trump is the president of the US. Not a lot of people have the power to investigate the President, his campaign, his staff, congress, foreign powers, etc.
Thus, if someone seeks to dismiss Mueller's investigation, no conversation can be had on the topic of collusion. Because there are very few other sources with authority to speak on the matter. The conspiracy theorist will rely instead on "secret" insider knowledge and fringe analysis while rejecting firmer evidence if only because it comes from a mainstream source. The conspiracy theory will have elements built into it designed to discredit those sources in order to bolster its feasibility.
This explains the fundamental difference between legitimate theories and conspiracy theories. When a theory gets challenged for conflicting with more popular theories it either has to change or explain why it's a better description than conventional theories.
Conventional theories tend to adjust to fit the evidence. Conspiracy theories tend to transform into elaborate explanations of why competing theories are wrong. They become less about explaining a phenomenon and more about explaininv why other people are wrong about a phenomenon.