Fundamentally because providing access to the Internet costs money (in hardware and facilities), and the bandwidth available to you is a function of the amount of money you spend on your connection to the public Internet.
Of course, even if you remove this barrier and buy yourself a dedicated 10 Gigabit fiber connection (if you’re interested, $5–10K per month on a 36 month contract) you can’t guarantee that the site you’re trying to connect to doesn’t have less bandwidth that you, or has a congested connection.
In reality, most users can’t use more that 50–100 mbps so it’s all academic.
In the wireless (i.e. cellular) world it’s more complex because of limited airtime resources, so users are limited to conserve these resources.