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Why do large RAM chips not have a serial interface?

I have a [PIC32MZ2048ECG064](https://www.microchip.com/en-us/product/PIC32MZ2048EFG064), a 64-pin 32-bit microcontroller with 512kB RAM. This is not enough for my purposes, I need to extend it by at least 32MB, preferably 64 or 128. I believe this means I can only use DRAM, because other types would not be available in this size. 

I read that devices in this family can access external RAM over SQI/EBI if you set up the MMU and TLB correctly. Because my device has only 64 pins it doesn’t have an EBI module, so I’m stuck with SQI.

I have looked around for RAM chips with a serial interface, but they only seem to go up to 4Mb or so, not close to enough. Is there a fundamental reason why this is the case? Is it because the serial interface is slower and the assumption is that if you have a lot of memory you need a faster interface?

I’m not looking for advice on a chip to buy, or for alternatives (I know I can get a chip with more pins, or one from the PIC32MZ DA series with 32MB on die), I just want to understand the apparent incompatibility between a serial interface and larger memory size.