1)Actualy you named the ONLY usable bug pokemons, 5 pokemons dont make it a useful atacking type. Scizor is also OU becouse he is a revenge whore Bullet Puncher....which is not a bug type move.
2) Its SE only against Dark, Psychic and Grass and is crippled by Stealth Rock.
3) 5 usable pokemons compared to the crapload of Water, Steel and Dragon pokemons that lurk around in today's metagame doesnt sound like good deal to me.
Scizor may be OU but the Bug type is down there in the crapper along with Poison as two of the weakest types in the game.
pokemons like buterfree are made only to help new players early game, if we ignore those useless ones you can see that most bug types are good, bug hit 3 important types for SE and most bugs have a way or another to kill steel types(superpower, CC, hp ground, etc), saying bug sucks because its weak to SR is like saying anything weak to rock sucks, the fire weakness is a let down but the bug type is nowhere near bad has you say, it get some useful moves like u-turn and bug bite(for LC bug bite is a awesome move) and even some reliable ones like X-scissor and bug buzz.
also, we got salamence, dragonite, altaria(UU) and latias for non-uber usable dragons, less then 5, for waters we got slowbro, milotic, swampert, quagmire and others(everyone know they are a bunch of water types), for steels we got magnezone, registeel, skarmory, foretress, scizor and metagross who are all here for the important dragon moves and/or specific stuff like scizor traping and spikes. the thing is that while they are not a bunch of usable bug types, they numbers are not so small compared to other types, they get SE on types used a lot(psychic-latias, dark-t-tar and grass-celebi for example) and resist useful types too(fighting and ground for example).
bug types are not fucking amazing like dragon by any means, but they dont sucks like you say they do. its just like dragon being weak to the so-common ice, steel being weak to the so-common ground and water-eletric, each of these types is weak to something common but pay off by doing something useful.
on a side note, the fact you said scizor main move is bullet punch made me mad, his main move is always u-turn since he is a scout that can also revenge kill AND trap(pursuit), the fact he got bullet punch just make him better, but his main move on a CB set is always u-turn.