Why are there no spaces in file and folder names?

I’m glad you asked! Spaces aren’t allowed in URLs, and instead get encoded, typically as %20. Say we had the following neighboring files:

contact me.html

Note the space in contact me.html.

If you uploaded the files to a server, the URL for the Contact Me page would be something like

Gross. Save yourself and your users the headache and use underscores (_) or hyphens (-) instead of spaces.

OK, well, why are they all lowercase?

This is considered a general best practice because some server file systems are case sensitive, and others are not. In order to avoid potential problems, the convention is to stick to lowercase.