Why are prepaid plans different from contract   postpaid? 

Prepaid does not require credit check which is why no credit is extended. That means you cannot get service until you pay for service and you cannot use installments to purchase a phone. AT&T has been no contract since 2015.

Why can you not get a plan that doesn't get slowed downon prepaid?

You can.  Prepaid has a 5 gig plan, a 15 gig plan, and a 16 gig plan, and an unlimited Max plan which has 22 gigs of priority data ( sort of a Walmart exclusive) 

     However, postpaid has a higher priority than all of prepaid. Business and firstnet have a higher priority than postpaid. 

Also why is the amount of benefits less but cost more?

Prepaid is definitely not more expensive. You must be reading it wrong. 

The cheapest postpaid unlimited plan is called the unlimited Value Plus. After autopay and paperless billing discount it is $50 before taxes. It is not eligible for any discounts or phone deals. It has no hotspot and no priority data. 

Compared to the prepaid unlimited Max plan which has 22 gigs of priority data and 10 gigs of hotspot, and is also $50 with auto pay discount. 

    So very close to the same price but the prepaid plan is a lot better. The better postpaid plans cost more. 

Was considering AT&T until i noticed this, unless i am reading it wrong.

You most definitely are reading it wrong. 

Prepaid is the answer for a variety of reasons, one of which is if a customer has credit issues since it's "pay-as-you-go".

Also why is the amount of benefits less but cost more?

Where did you get that idea? Prepaid plans cost less than postpaid.