Getting ghosted by big company?

I had an interview in march --> one with two scientists/hiring managers, following week was phone call by HR/talent acquisition on 3/16/21, she told me she'll let me know the next stages for the final interview within few weeks. Well, she hasn't told me any update. 
I have been applying to other jobs and face a similar fate. Why do these companies ignore your application? I would give them about a month before moving on but at least have the courtesy to inform candidates as to whether they are selected or rejected. This ruins their chance of having such candidates reapply for future positions within the same company again. I get that things are busy on the other end, but sending out an update or rejection/selection won't do no harm. It just shows how the employees get treated like dirtbags.

A Hunch2021-04-07T15:25:02Z

They didn't ignore your application.
They brought you in for an interview.
There was a follow-up conversation.
They said they would call you in a few weeks = it's still a few weeks.
They probably haven't made any decision at all

"I would give them about a month before moving on"
- until you have an accepted offer, you always continue looking for a job.
- if they eventually call you back, then you can decide at that time if you are interested in proceeding. 
- if you don't want to re-apply because they didn't treat you how you wanted to be treated, then don't apply.  Most people don't have this same view as you do.

Not getting a letter telling you, you were rejected is not being treated like dirt.

With your unreasonable expectations, it sounds like the companies are better off not bringing you into the mix.