Two reasons
1) Because, Chrome is ready to do something with that tab: browse the web. Do this, create a browser that doesn't use much memory when a tab is idle. Switch back to that tab and interact with the page or go to a new page. You will experience latency. It will feel slow. That's why the tab uses a lot of memory. It is staying ready at all times to be interacted with by the user and the network.
2) The real reason: Chrome either has no memory budget or the memory budget was set to 100 MB. The engineers decided that some amount of memory usage was acceptable and that's what you get. If a new budget was created, say 10 MB, then what you would see is some very clever engineering to overcome the problems in the first reason I gave. Chrome would be responsive at all times and use very little memory per tab, but at great engineering cost. It would be very difficult, but interesting work.