Say I am at a spot where I do not have signal from the carrier I am subscribed to, but I can have signal from another one. Why should there be any friction in b

Say I am at a spot where I do not have signal from the carrier I am subscribed to, but I can have signal from another one. Why should there be any friction in buying individual minutes/megabytes from that other carrier? This is obviously impossible when SIMs are tied to a specific carrier, but in fact, why shouldn't mobile phones offer the option to pick any available carrier depending on which one is proposing the best rates? Also, about swapping cards: dual SIM telephones are fairly popular, which illustrates that many people are using more than one SIM and do not like to have to switch (because they want, e.g., to be able to receive their texts on both). This is just a temporary solution, the right one would be to have one SIM but tied to multiple providers.