If you think about the signal carried by a guitar cable, it is actually very low. The vibration of your strings excite little magnets we call pickups, and these small vibrations create a tiny current that is carried to your amp.
Speakers, on the other hand, are driven with powerful amplifiers that send current to a large magnet to move the speaker cone. As such, Speaker cables are made to carry more current.

Physically, the difference between Speaker cables and Instrument cables lies within the internal construction. As guitar cables have such little signal to work with, they also have to worry about picking up other weak signals like electrical hum and radio frequencies like bluetooth or phone signals. To overcome this instrument cable manufacturers will include electrical shielding to protect the inner wire (which carries your guitar tone).
Better Instrument cables will have quality braided shielding and the cheaper ones will have foil - but speaker cables often just do without it or minimize shielding.

Speaker cables carry a lot more signal as we know, and if we think of the signal as a water current (in place of the electrical current) then it makes sense that a large volume of moving water would react less to outside influences than a small trickle. Your Instrument cable carries a small trickle - so it uses multiple layers of shielding to protect it from being influenced by outside noise, but your Speaker cable has nearly no issues with such small interference - it’s like a drop in the ocean.