Being careful and being successful are two different things, especially given how hard it is to do an exhaustive search over patents to ensure that no, no claim in any patent filing touches your code, especially given that there are regimes where patent filings for software get pretty much a rubber stamp and are based around "first to file" even if there's prior art.

That's what patent trolling is

Basically, unlike copyright, you don't even have to know that someone else patented something in order to infringe on a patent and have them come out of the woodwork later. It's just even more expensive if you do know.

The patent system was not designed to have every tiny little technique patented, and this is its failure mode.