This is exactly the problem with WiFi. It is a "shared" media. What does that mean? Let me explain.
When phones came out long ago, they were "shared" lines (called party lines).
Party lines were one phone number shared by a group of people, (it would allow people to split the cost of the a then expensive new humanless patch phone system).
But how it worked was if you picked up your phone, and instead of a dial tone you may hear someone else using it (aka someone from the party).
If it was busy you hung up and tried again later, since only one call could be done at a time.
Later on when "switched" technology got cheaper everyone got their only private line.
Your data plan is like "switched" phone systems.
Your WiFi part of you phone, computer, I-whatever is like the "shared" party line system.
So on a "shared" system everyone is trying to use the same line at the same time. It is amazing that it works as well as it has because it is limited. Limited by bandwidth called bits per second or bps. On the B and G WiFi system it is limited to 11 million bits per second. But if 2 people are on it is split in half 5.5 million bits per second. If 11 people are on it they each get 1 million bits per second, This is not exactly technically true of the math but you get the idea. Now A WiFi has 54 million bits per second, put 54 people on it and your back to 1 million bits per second. N has 54 to 600 million bits per second, and that sounds really awesome, but it problem is that you have to have lots and lots of access points (access point is the radio transceiver for WiFi) very close together. Imagine have cell towers every 1000 square feet (can you say eye sore). There is a lot more to it, but that should help.