The usual excuse for not open-sourcing is that commercial products aren't developed in a vacuum. Flash, for instance, supposedly included a bunch of codecs that Adobe didn't have the license to redistribute as source.
At a bigger company, there are presumably dependencies on in-house build systems, standard libraries, etc. Maintaining open-source versions of those is unfortunately a non-trivial task.
You just make all the dependencies open source at the time the rest of it becomes open source, then you can keep the updates in-house and the community can update the dependencies if necessary and willing
That still assumes the dependencies are first-party. Some well known CVE examples have been how much both macOS and Windows had internal dependencies on Adobe code. Surely Adobe still has a commercial interest in their code even as/when both macOS and Windows dropped the features that relied on those dependencies?
(For other examples, the game industry is full of well known third-party "middleware" like Bink, SpeedTree, and much more because those middleware like to force their logos into places for advertising as a part of their licensing terms. If Windows had opening or closing credits it might be a surprise how many logos might be forced to show up in it.)
At a bigger company, there are presumably dependencies on in-house build systems, standard libraries, etc. Maintaining open-source versions of those is unfortunately a non-trivial task.