First is security, if current asset based on linux is vulnerable and you make it public consequences are obvious. Or even your system is vulnerable itself because of modifications. Or 0day that will reveal afterwards. And your product is based on embedded systems. I understand it's your choice, but it holds the development.

Second is competitive advantage. If you have to release modified system, that you use in your product on day 0, when you started, it interfere your business. Again it's your choice, but it could hold the development. So reasonable alternative would be good.

If you choose to use code that is under the GPL or similar copyleft licenses, then you have to play by the rules and provide people you distribute your software to a way to get the source code.

Yes, I understand

But it's much better if comment by zmmmmm is correct and it only touches kernel modifications.