People in general don't like being told they're wrong. This means that arguments that challenge status quo get suppressed. Science isn't magically immune to this just because you add a label "it's scientifically proven, bro!". Therefore, a lot of research on controversial topics can be safely discarded simply because people doing the research have lots of reasons to be biased. This is especially relevant in social sciences, because it's a bottomless pit of controversial topics, and has almost zero possibilities of repeating an experiment. Sure, we have 50 years of research proving that children are blank states, but it's important to remember that eugenics were deeply rooted in science too. It's just that different societal attitudes expected scientists to come up with different scientific results, so they did. Think of a society-wide version of corporate-sponsored research centers that are expected to massage the results until they match the desired outcome.

Another thing is that sometimes it might be beneficial to believe something that isn't true. If you knew for a fact that tomorrow 99% of population will suffer extremely painful death then from the point of view of an individual the correct move would be to commit painless suicide, but the survival of humanity relies on everyone believing they'll be all fine. This is obviously a caricatural example, but there are lots of such lies that keep the society going, and "we're all equal" is just one of them.

Personally, I find it extremely difficult to believe that we'd be born equal, because evolution works only if some individuals are better than others, and I strongly believe that evolution is a thing.