Telecom company does not have any data . It charges you for the amount of data which has been transferred from the server to your Device. (Smartphone ,Laptop ..etc) .Now lets say you want to download a movie which is 1 GB (Gigabytes) in size .
For that you will switch on the "Cellular Data" option on your Device and then click on the download link to download the movie.
Here , Telecom Company connects you to that Movie Server through its Network so that you can have that 1 GB of File.
You must have seen these Towers :
Your Device (Smartphone , Laptop with Data-Dongle etc..) is connected to this tower via a Radio-Link , which is invisible to your eyes. You get connected to enodeb if you have a 4G Device (architecture is little bit different for 2G and 3G).
This in turn is connected to other Devices as shown in the image below .
A Telecom Operator has to install all these Towers and Devices to connect you to the internet (Internet / Fixed IP Network as shown in the picture).
These devices and Towers have to be operational for 24 hours to give you 24/7 services . To install and operate these Towers and devices ,a lot of cost is involved. Electricity is required to run the network, which we get from Electricity Service Providers / Diesel Generators / Solar Panels etc .
These resources are limited. Also for more number of users , more towers are required , because every device has a limit. So, when more number of users get connected to one tower , the average speed per user served by that tower drops , resulting in customer dissatisfaction.
You see , we have limited resources and have some Bottlenecks , therefore Telecom Operator does not provide Unlimited data (on Full Bandwidth)It sometimes provide Unlimited data , but in that case it has limit on the Download-Bandwidth.
Thanks for your feedback