Because, to be honest, whats the point? We can pretty much determine the composition of the atmosphere with spectroscopy, and we can't land without being crushed, boiled and dissolved at the same time. If we go to Mars, we can potentially find things on the surface (eg interesting geological formations) much faster than a rover could, and potentially run more in-depth scientific tests on what we do find, rather than just what we can send on a single rover

We shouldn't set foot on Mars for a long, long time. Read the bit about contamination.

https://idlewords.com/2023/1/why_not_mars.htm

> We can pretty much determine the composition of the atmosphere with spectroscopy

I don't believe (could be wrong, not an expert) that spectroscopy is sharp enough to tell you about the structure of complex molecules in Venus's upper atmosphere.

Please read the article.