There must be some factor other than gravity that contributes to an atmosphere’s pressure level because Earth and Venus have almost the same mass.
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First off, to correct *many* wrong claims in answers here. Temperature of an atmosphere *does not* affect pressure directly. If you have a sealed box of gas and raise its temperature, yes, the pressure will go up. However, an atmosphere is free to expand outwards – that’s exactly what happens if you heat it up. Instead, the pressure is just the weight of everything above you – if you heat an atmosphere it will readjust its density to make this true (see: equation of hydrostatic equilibrium).
As to why Venus’s pressure is higher, in one sense the answer is simple. Because there is more stuff in the atmosphere. Why that is, though, is more interesting. The main answer seems to be runaway greenhouse effect evaporated the oceans, which in turn [turned off plate tectonics](https://www.space.com/venus-runaway-greenhouse-effect-earth-next.html). Once that happens, carbon that would have gotten sequestered from plate tectonics stuck around and got released into the atmosphere. Way, way more CO2 in atmosphere means higher pressure.
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