Sammao you are incredibly off the mark for Forretress, access to Toxic Spikes and Rapin Spin keep it A tier. Its utility is not as immediately apparent because it doesn't check as many things, but it is most definitely an A tier Pokemon because it provides unique and invaluable team support.
Kabutops is ok to be at B mainly because Groudon can switch into it pretty easily and that neuters it entirely, but it's very, very good and it's top tier B. Never simply just say 'look at the other Pokemon in this tier and you will see why' what kind of logic is that? Please don't be so nonsensical.
Bisharp is ass lol. I would argue it's even D worthy.
Mamoswine is better than Bisharp but almost every single Arceus forme can either wall it, kill it, or both. Even Grass Arceus and Steel Arceus, which are technically 'weak' to Mamoswine, both beat it as long as they don't switch in on Icicle Crash and Earthquake, respectively.