SRAM is usually produced on the same wafer as the rest of the logic. SRAM on an external chip would lose many of the advantages without being significantly cheaper.
Yes, the limiting factor for bandwidth is generally the number of pins which are not cheap and you can only have few 1000s in a chip. The absolute state of the art is 36 Gb/s/pin[1], and your $30 RAM could have 6 Gb/s/pin[2].
