Long story short, profit.

The not so short versio is that each manufacturer along the supply chain decides for how long they want to provide updates, the following manufacturer can’t provide updates that the previous manufacturer doesn’t. And so it goes until you get a phone that has 3 years of software updates and 5 years of security updates and that’s it.

For example, you might buy a phone from google, they buy electronic parts from maybe Qualcomm and a few others. Say Qualcomm decides that it won’t be profitable to provide driver updates to the SoC past 7 years and critical security updates past 10 years. At this point even if Googke really wanted to, they can’t provide anything past that as they would not be able to update the Qualcomm driver. But google has otger providers with maybe shorter support windows, abd they have their own costs and updates to make. So Google decideds that for them it won’t be profitable to provide support past 5 years and security updates past 7 years. So now you get a phone which will be unsupported after 7 years, and here comes the big difference between phones and most computers. Phones have a locked bootloader and usually a custom SoC. Locjed Bootloader means that you can’t just install another OS that might still provide some support, and the custom SoC means that any driver support will have to be only for this specific device.

So with an old laptop you might have a custom motherboard, but the cpu, gpu, ram, wifi, sound, etc will all be off the shelf, so if some linux developer makes some sort of driver support for your wifi card, it will also apply to all other laptops with that wifi card, but for a phone this won’t work, let alone even be viable since the bootloader is locked.

Simply put, no right to own and profit chasing means that no one really cares if your phone is a security and usability nightmare after a few years.