I haven't heard about companies decompiling programs of other companies
and using the code  to put in their own programs. I think it happens for
small pieces of code, but not for whole applications. If they do that,
it would be obvious that it is (almost) the same, and you could sue them.

Writing code  is only a small part of the development effort for any significant
program. Requirements-gathering, design, testing, documentation, distribution
and life-cycle outweigh the coding cost.

Additionally, writing code is really quite simple, compared to the other phases.

What this means is that if someone chooses to copy your program, they probably
won't bother to look at the code, they'll just copy the idea and code from
scratch.

Perhaps a good analogy is the family car. Line up the basic products from
different manufacturers and it can be hard to tell them apart. Clearly there's
a lot of copying going on somewhere. But look closer - not a component in
common between them (excluding third-party subsystem components - e.g. tyres,
alternators - and OEM - e.g. rebadged Mazda 323 in Ford livery).

It's the concept which is worth copying, not the code itself.