Why don’t companies like me working another job?

Lots of potential reasons.  A couple are because they want your priority to be the job you work for them and want you well rested to work when it is time to work... and  because working elsewhere may reflect poorly on the company as you are a representative of the company regardless of whether or not you are at work. 

It affects your availability to be flexible in working for them.

It all depends.  How do they even know you’re working another job???  Or is it their policy for some reason?  They wouldn’t want you working for a competitor or have the other job affect your work for them in any way, such as hours.