5w/kg for an hour doesn’t win GT stages. The top climbers in the world can do over 6w/kg for half an hour on uphill finishes, but those aren’t isolated efforts… those usually come after a hard day with several uphill efforts before that. That’s a completely different kind of effort.
For example I can 100% vouch for Lionel Vujasin (One of the world’s top Zwifters), I’ve seen him race live twice and his Zwift numbers are 100% legit.
Understand that just because both are basically riding a bicycle, doesn’t mean it’s the same discipline.
Look at Chris Hoy… one of the very best track sprinters ever. Yet he probably couldn’t win a GT sprint stage if his life depended on it. Visa Versa Mark Cavendish…
Anyway, to the issue at hand, first of all: ride in the correct category… Looking at the numbers you mention your FTP is probably arround 3.5-3.6wkg, so you belong in cat B.
Next to that: where are you checking your results? If you’re talking about results straight from Zwift, then you should ignore those completely. If you want proper results that filter out cheaters & people in the wrong category then zwiftpower should be your source for results.
Last of all: cat D races, in all honesty, are a ■■■■-show. There’s usually not a lot of riders, and out of the riders that are there 75% is in the wrong category.
Most of all… I think your biggest problem is not understanding the game mechanics. I see it all the time: people pushing out higher numbers then myself getting dropped from groups like bricks and it’s always down to the same thing: they don’t understand how the drafting mechanic of the game works.
I suggest you join some group rides (look for 2,0-2,5ish rides or something) in the next couple of days with 1 goal only: get a good feeling for how drafting works. See what happens when a group hits an up- or downhill stretch (see how the shape of a group changes from blob to strung out), see what kind of effort it takes to just sit in the bunch and most of all also get a feeling for what happens when you do a pull in the front and see how fast and hard you slide back into the group as soon as you let off the power…
Last of all: understand that the way Zwift racing is organised (in wkg cats), all cats below cat A heavier riders are favored. Your 78kg is not the very lightest but it’s definitely no where near heavyweight.