But running a SC does have overhead and risk. It seems obvious to me that companies will use the mainchain if possible (that is, it scales and can capture the transaction).
There are significant number of people who don't want the main chain to scale, for various reason.
Sidechains provide those people both with an argument against scaling the main chain, and it creates a group of people with a financial interest in preventing the main chain from adopting new features which might render particular sidechains less necessary.
The latter part wouldn't be such a big deal if Blockstream wasn't so closely connected with Bitcoin Core development.