Why is my internet terribly slow?

Yes like many, many others I am having trouble with my (wifi) internet speed through AT&T. My plan (which is an awful plan to begin with) has me paying about $60 a month and only offering me a download speed in a range of about 3-6 Mbps. However, in actuality, I will only be able to have about 1 Mbps when no other devices are being used (1 Mbps=.125MBps=125KBps. A typical photo is about 100 KB, some can be 1 MB or more). It is not a problem with the modem, my device, or any environmental disruptions, so why am I not being given what I'm paying for?

Moreover, I have had this plan, or similar ones, for years, but now there are offers advertised, available in my area, that allow a download speed of up to 50 Mbps for a price of $40. First of all, if AT&T can allow for a plan with almost 10x my available download speed for less than I am paying, why can't I at least get what I am paying for? Second, why is it when informed of any problems, the company will only either schedule an appointment for someone to inspect the modem, or just send a new modem without fixing the problem? Finally, why the helI am I paying so much more for so much less?