A silver 1 player gives him more time to aim so it's easier to hit him.
A silver 1 player doesn't dodge his bullets by spamming ADADAD and crouchwalking so he's a much easier target and requires much less aim adjustments.
And moreover, in 5on5 there's more hodling spots from positions that favour your enemies, so if you just run'n'gun like on a public server - you will be taken roughly from behind or from any hiding spot which you can't check fully. Plus, with at least a bit of teamplay you will face at least a couple of enemies and one will help another to fuck you in the ass.
And no it is not because of confidence, you don't know if your enemies are good or bad before the match, it's not like you see NiKo and coldzera in the opposite team and you think "omg I'm fucked" and your confidence goes low, lol. Unless you're just afraid of playing 5on5 and you think you can do nothing in 5on5 - then yeah it's about confidence, but I doubt you really think you suck at teamplay, so it's exactly what I have been talking about - your public style does not work against a TEAM.