Why do you think that scientists discuss manned missions to mars but not to venus?

Scientists discuss manned missions to Mars because it would be much easier to send humans to Mars because the conditions on Mars are much less extreme than those on Venus. For example, surface temperatures on Mars range between -125ºC to 35ºC while those on Venus range between 450ºC to 475ºC.

Venus is a super hot planet. People wouldn't be able to survive there even with space suits. But with mars its way different. People could survive and mars and gather resources as long as they have some source of oxygen. 

Answer they are produced by collision of tectonic plates;