I'll tell you why. Because different people in different situations are equally beneficial to Canada in different ways. Yes, a student who has been in Canada for a few years already is a good prospect, because they've integrated (hopefully) a bit, and have a network (ideally). Those aren't guaranteed though. You could keep to your group of Chinese/Indian/Mexican friends and just not explore anything if you wanted to.
In the same way, an FSW candidate is also a good prospect, because while they may not have been living in Canada up to this point, they might have many more years of actual practical work experience over that student. They may also have a higher language score, already possess a Master's or PhD from overseas, and may already have a relative living in Canada. Or they work in a very in-demand NOC category overseas.
That's exactly why there is a scoring system for EE. Since this whole system was launched in 2015, if candidates inside Canada were better prospects, they would've just been given CRSs of like 1,000 and FSWs would be given, say, 200. But no, instead it's a combination of factors that gives you your CRS, and you have to work on all of those factors to get past a certain point to be drawn. Whether you're CEC, PNP, FSW, FST... whatever. We're all in the same boat.
It's not as easy as saying PGWPs should be the only ones invited because they're already there. This old debate of who is a better candidate is so tired now. And I guarantee you Mendocino and the IRCC aren't busy shutting down EE. In fact, they're actively proving the opposite.
From what I gather from the discussion in this thread, according to CEC candidates foreign work experience and foreign education is worthless compared to Canadian education and Canadian work experience in Canada and even if a FSW candidate lands in Canada, they won't be able the integrate or find jobs because they have never lived in Canada. Wonder what the FSW candidates who were getting PR visas since the last 5 years are doing there now.