Because it is a single use licence.

Now if 2 people have the same game in Family Share, then 2 people can play it.

Originally posted by chezcheney13:

Family game sharing

Why can me and my family not play the same game at the same time with family game library?

Can we go through a real world example of how a Steam Family might share games?

Of course! Let's say that you are in a family with 4 members and that you own a copy of Portal 2 and a copy of Half-Life. At any time, any one member can play Portal 2 and another can play Half-Life. If two of you would like to play Portal 2 at the same time, someone else in the family will need to purchase a copy of the game. After that purchase, there are two owned copies of Portal 2 across the family and any two members can play at the same time.

In this example, if your family choose to not buy a second copy, you can play any other game in your library while waiting for your family member to finish playing your copy of Portal 2.

https://help.steampowered.com/en/faqs/view/054C-3167-DD7F-49D4#frequent

Originally posted by Vixu:

-snip-

Having trouble with the spam I take it?

Thanks!

Originally posted by chezcheney13:

Thanks!

It is essentially the same as handing a cartridge to someone so they can borrow while you are not using it. Some will say there is a difference between that a digital license and what not but it is working as intended. The bright side being your family doesn't need to buy the game again in order to play it if you have it.