It's a lot easier to pop a cartridge in than to disassemble the case to get at the JTAG pins.

I don't think there necessarily ever were pins. JTAG on a production line is normally hooked up via a bed of nails.

These are called pogo pins

https://en.m.wikipedia.org/wiki/Pogo_pin

these care called a bed of nails

https://en.wikipedia.org/wiki/Bed_of_nails_tester

In addition to what the others said regarding disassembly, it's a lot easier to distribute (and train remote technicians to use/maintain) a test cartridge to service centers around the world than to set up the $xx,000+ bespoke bed of nails test fixtures that you would use on the production line.

JTAG doesn't really exist on the N64 hardware, it's only present on the CPU and the pins are hardwired to VCC/gnd.

Even if it did work, it wouldn't be sufficient to test the rest of the system, which doesn't have any concept of JTAG.

The other major chip (RCP) does have an undocumented test mode, it allowed the verification guys at SGI introspection into various registers/state by exposing them onto the cartridge bus as gpio. I don't think this is written down anywhere though.