Because the internet works with packages that have to be received and assembled and you also have to share bandwidth with other things that need internet. Cable TV on the other hand is just a constant signal being sent, which the cable box only has to decode
cable tv is more basic, each channel provider is on a different frequency compared to 1 with internet to send any kind of data back and forth. each frequency is given an appropriate amount of bandwidth for the channel provider to encode all the streams and channel information.