a few points.
1) you say “when you spend 40 minutes on barely understanding a daf a day how can you say you finsihed shas when you barely read the english and you didnt even understand every other line.” speak for yourself. You say ” i tried learning daf hayomi and it took me 2 1/2″ Not everyone struggles so much with the first daf of berachos like you do. Though Kudos to you for persevering. Some people can get a basic understanding of most dafim in 40 minutes. (though retention is a problem)
2) “wouldnt it be better to send 40 minutes everyday learning a mesechta roerly with tosfos and meforshim.” Yes, I don’t think anybody disputes that.
The key though is every day/. for many people such “every day” learning quickly becomes not every day, We just concluded a busy yom tov season, I doubt many who learn “40 minutes everyday learning a mesechta roerly with tosfos and meforshim” in fact did it everyday, then there are weddings, busy with kids , tired from the clock change
3) “i dont understand the point of saying you learnt the whole shas when you understand a tenth of it.” a few points a. It creates a sense of accomplishment that drives the daily learning that takes no breaks for yom tov, weddings, or clock change. and b. Many (if not most) who have done daf yomi have at least a passing familiarity with shas. I think that is a good thing.
hope this helps you understand the reasons. If you dont think these are good reasons, thats fine of course.