Zygarde can't sweep well because it's weak as shit, it's either you play him like an inferior dragonite, garchomp, or hippowdon/landot/gliscor, it's movepool is not as good as it's hyped up to be
You sound like you expect Zygarde to do everything, and better than everything else in that regard. While Zygarde certainly can't do everything well, no other Pokemon can either, and Zygarde definitely has valuable niches that can't be discounted.
bulky dd is better done by dragonite
You say that bulky DD is better done by Dragonite, but I find this to be simply untrue. While Dragonite has immense capabilities with Multiscale, it's not like Multiscale makes it into some magical tank that won't ever die, as this is easily handled. On the other hand, Zygarde has an undeniably awesome typing both offensively and defensively, and it has defensive stats that make even Hippowdon jealous, so Zygarde's bulkiness is not something you can argue against.
offensive dd just pales in comparison to salamence and lum dd nite, espeed is only good if it hits off AT LEAST 90 atk with STAB or 120 without
Neither Dragonite nor Salamence have STAB Earthquake, one of the most powerful tools in the metagame. Think of the STAB boost like a free attack boost. Wouldn't Salamence and Dragonite just love that? Besides, Salamence and Zygarde are almost too disparate to compare. The bulk difference, ability difference, and movepool difference (namely Zygarde's access to E-speed) is what puts them in almost completely different fields. E-speed is also not as weak as you think: Scarf Rotom, something that would normally threaten Zygarde's sweep, can be hit for roughly 47% to 56% damage after a boost. That's low enough that you can rely on teammates to weaken Zygarde's counters to the point where Zygarde is no longer hindered by their presence.
coil is just a shitty gimmick that doesn't patch zygardes 3 weak spots well enough: attack, speed and sdef, amd glare honsetly is asking for a sub/ electric (like rotomw) mon to set up on you
Hating on coil and glare? Take a step back for a second. If you're implying that coil doesn't make Zygarde into an incredibly fearsome threat, then I think there's an issue with how you're thinking. With Zygarde's stats, a single coil makes it into one of the most fearsome tanks I can think of. It lets it reliably use Stone
Miss Edge to take out Togekiss, Dragon Tail to mess up Skarmory, and most importantly gives it an extremely viable fighting chance against the hordes of physical attackers present in the metagame. A Pokemon with Hippowdon-like defenses is not going down easily after a +1 defense boost, and a +1 attack boost just makes it that much more threatening.
Glare, on the other hand, is a great way for Zygarde to deal with its <100 speed. If the opponent sends in a counter not an electric type, you're paralyzing it and giving yourself a good chance to not only outspeed it but also knock it out with a +1 EQ or Outrage the next time they switch in. I think it's extremely potent in the right hands.
and no zygarde is not a better sweeper than garchomp, subSD and yache chomp were top class sweepers in OU and in general garchomp should dilute zygardes usage just because it's so fucking good compared to zygarde and I just cannot see this thing being anything more than flygon 1.5
No. Zygarde is not worse than Garchomp. They are two different takes on the same typing, and two different playstyles as per the arguments I've outlined above. Yes, Garchomp is a fearful sweeper. No, Zygarde cannot do many of the things Garchomp can do. But Garchomp can't do many of the things Zygarde can do, and if you think it can, you're missing out. Again, stop comparing Zygarde to Garchomp, and DEFINITELY stop comparing this thing to Flygon. That's just like trying to compare Rayquaza to Altaria.