Many customers wants to buy a product, but wants to be able to maintain it themselves. Or want at least have that as an option.

A number of customers may even buy a compiler (unless the project was developed with free tools) just to make sure that in case they need to take over, they will have everything they need.

It very much depends on how much money the customer puts into a project, and how many years of profit they hopes to be able to gain from it. And if they not just buy software, but including hardware, they know that they need to keep that hardware filled up with working firmware for as long as they are interested in building the hardware.

If the customer investment is $1,000,000 or $10,000,000 or §100,000,000, then they tend to think quite a lot about the full life span of a product.

When employed, most companies have very strict specifications about what languages and tools that should be used. Not just what language, but also what manufacturer and what version and what linker and what source-code versioning application and what test tools, ...

And there may be extensive documentation about how the code should be modularized and indented and naming schemes, ... All there to give a uniform process where code written by one developer will be readable and compilable and linkable by another developer and where someone who reads the code will not be able to see who wrote the individual source lines without looking at the source repository annotations.