It's not that simple. What if some of it is proprietary? What if I'm not allowed to submit code because I need to be anonymous so reviewers can maintain impartiality? What happens when one of the upstreams update and breaks my code? Do I need to keep it updated? Forever?

At my institute at least, scientists are required to maintain everything that is necessary to reproduce a result for at least ten years. That includes all the data and the software used to produce the results. It's not an easy job, but it's important.

If your institute also mandates they make the data/software publicly available, then that's definitely the exception rather than rule. Also must be hideously expensive.

It almost never happens that a paper I read actually comes with usable source code.

Then your results are not reproducible and your conclusion is suspect.