False Consensus effect
Etymology
From Latin consensus, "agreement." Psychologists Lee Ross, David Greene, and Pamela House named the effect in 1977, after finding that people who agreed to wear a sandwich board around campus assumed most others would too — and that refusers assumed the opposite.
noun · /FAWLS kun-SEN-sus ih-FEKT/
The tendency to overestimate how many other people share your beliefs, tastes, and choices.
What it names
Why "everyone thinks this" so often means "I think this" — we treat our own view as the silent majority.
Example
Trapped in a false consensus effect, the founder was stunned that "obviously useful" feature drew shrugs from actual users.
Related idea
Spotlight effect overrates how much others notice you; the false consensus effect overrates how much they agree with you.