because I like the UI and it's much easier to use for me
the last time I tried FDM, several times actually, FDM was always blocked my by malware filter and my AV, and the UI is a bit confusing. Not really better than eagleget. Perhaps they have the same speed
Eagleget also uses less RAM if I recall correctly
maybe FDM can suit you better but for me eagleget is more comfortable
I think all of them can work well with firefox quantum, not so sure
Most video downloads, or embedded videos can be detected from within the browser itself by inspection tools. I never used any download manager apart from JDownloader for automation reasons, it can download videos from every non-redirect website, crawl for links hidden behind the shortened urls full of ads, resolve or simplify captchas and download multiple / dissected downloads that, in case of not having JD, you'll need to click and download 1by1.