You cut off the "if I haven't touched the tab in an hour" part.
How many times does that pretty-narrow situation happen to you in a day?
If you had such a memory-saving mechanism, boosted by prefetching the tabs adjacent to my current tab, it would barely cost me any seconds in an average week. Running low on memory is a big hassle taking a lot of seconds to deal with.
I'm not. I open dozens of pages simultaneously routinely. 1 second here, one second there, it adds up.
Sure, it's not the end of the world, but those differences were why Chrome was better than Firefox.