Who's to say Kakashi has any knowledge about the tablet tho? Or that he knows the seal to enter the room that only Sasuke actually shown to use while Orochimaru who studied everything probably should have know. More than likely takes the sharingan and a seal.
Suigetsu said you need a jutsu to open the door and Sasuke has his Sharingan active but has it off after going inside
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Kakashi wouldn't know about the MS weakening their sight unless his was as well.
Read what I said above, there's no way that has been shown, suggested or exampled how Kakashi would know the MS weakens an Uchiha's eye sight unless his eye sight was also effected. Plus Kakashi didn't use his MS as much nor as long as Sasuke and Itachi. Kamui is a quick attack while Amaterasu and Susanoo requires the MS to be used longer. Amaterasu requires you to focus on what you want to burn (target moves and you chase them with flame like Itachi did), need it on to manipulate the enton or to put it out. Susanoo is the work for obvious reasons.
Kakashi used about 5 Kamui's before the War arc then like 6-9 times in the war. Sasuke having his MS active takes away some of his sight, using MS jutsus only do it quicker. Sasuke and Itachi activated their MS more than Kakashi. Obito is another case.