I’m new to CAD, what is the simplest software to use? SpaceClaim looks very “novice” friendly but seems like no one uses it, why?

if you can get a hold of a solidworks license, use solidworks above anything else (unless its catia, but thats rare). else, use autodesk inventor. if you have a .edu email, all of their programs are free to use. just stay away from that terrible piece of software called fusion360. companies don't want to invest in it, it takes way longer than it should to model stuff in it, and there are multiple other issues with it as well.

You can use any CAD software available, I am using Fusion 360 its simple and can give you high quality results, it has Part, Assembly, Stress Analysis, Assembly /Dis-Assembly Animation, Printing, CNC machining ((ALL IN ONE)) solution

You are looking for the Holy Grail of CAD. Once it is created that simple, friendly program will become the standard software which everyone one uses.

Right now I'd say the market leader is SOLIDWORKS, but there are dozens of 3D design programs.
The right one for you can only be found through trial and error, but you can help narrow down the list.

1. What types of parts/models do you aspire to build?
2. Working alone, or with a group?
3. Any need to export files for other uses (graphics, CNC, 3D printing, other CAD programs...)
4. What is the budget for the initial expense, then yearly "maintenance fees"?
5. Do you even need to upgrade each year? (usually because you are working with others who have upgraded)
6. How do you learn best? Books, videos, classroom? How much time and money are you planning to put into learning CAD?
7. Are you building CAD models for fun, or CAD models for actual production and manufacturing?