Why can't we say Shma all day?

The Mishna at the beginning of Brachos says that Rabban Gamliel says that you can say Shma all night. He says that when the Torah says you say it b'shachbecha he doesn't mean only when people go to sleep but any time that people are asleep. However, on 9b all the Tanaim agree that you can't say Shma as long as people are awake and on 10b the gemara says that we pasken like R' Yehoshua that you can say until 3 hours. Why for zman schiva is there a machlokes if it means as long as people are sleeping or if it means when people go to sleep but for zman kima the only machlokes is if we go after when most people get up or when the last people (kings) get up but definitely not for as long as people are awake?

I guess that the answer is from the words that the Torah uses. Shechiva could either denote when people lie down to go to sleep (R' Eliezer) or when people are lying down (R' Gamliel). Kima means when people awaken
not when they are awake. I guess that's the pashut pshat in the words. My only slight problem with that is that Beis Hillel on the mishna at the end of today's daf says that when the Torah says b'shachbecha u'vkumecha it means shas shechiva and shas amida. If that's what it means then it should be all day. I guess that even though Beish Hillel says that he really means shaas kima and not shas amida.