[Telepathic Intruder]

Venus has substantial atmosphere loss, similar to Mars which once had enough air pressure to support liquid water. But since Venus has so much of it, it will take a long time to deplete. To add to your point, however, the moon Titan has an atmosphere thicker than Earth's, even with the much lower gravity. The difference is the amount of solar wind. Titan only gets about 1% that Earth does.

Earth's magnetic field keeps most of the solar wind from interacting directly with earth's upper atmosphere, which would ionize it and blow it into space similar to what they think happened to Mars.

[SunkenCiv]

There is a difficult-to-kill wish that Mars was once "Earthlike", but it's apparent that it has never had much atmosphere. Venus must get hit very hard by the solar wind -- the atmosphere must have been even more massive than it is now, unless of course Venus migrated into its current orbit, which isn't terribly unlikely, given its lack of tidal lock with the Sun (Venus has a slow retrograde axial rotation).

[Yardstick]

Thank goodness for Earth’s magnetic field. We just about ought to have a holiday recognizing it.