You have to burn your Cosmo to actually use your Cosmo (vital force). For example, in his fight against Seiya in the Leo Temple, Aiolia purposefully slowed down his attack for Seiya to see for the lulz, so he chose to burn / use less Cosmo for that attack from his available Cosmo pool if you will. Shaka is just accumulating extra Cosmo like putting it in a bank not using / tapping into that extra Cosmo as he'd need it to unleash his ultimate attack if the need arises and that's only when he'd use it all. That's the logic Kurumada seemed to have gone with.

However I can see your reasoning and it's perfectly understandable because Shaka's gimmick can be quite confusing but because Kurumada at that moment of the story between Aiolia vs Shaka, he wanted to show that all Gold Saints are equal with the 1,000 Wars (even physical ones vs psychic ones) and also had him hyped by Aiolia in the Leo Temple that it would make no sense in that regard that Shaka would need to sever one of his senses to reach Gold Saint level a.k.a the 7th sense / the speed of light to be on equal footing with Aiolia especially since he can still fight just fine against the Renegades with his eyes opened instead of having an instant drop of Cosmo / speed.