I've had this thought in the past, but upon further research, on a smartphone scale it runs into a few issues.

A PC has relatively divorced components because it has the space to accommodate slots, but that already goes out the window for laptops; the best you get is RAM and maybe MXM. People generally want slimmer and more battery-dense products, so motherboards with everything soldered can manage to be absolutely tiny (see: newer macbooks) compared to ones where everything is slotted. Newer smartphones don't just last longer on battery because they're bigger; much of the internal volume of a smartphone is now to hold the battery as everything around it shrinks. Modular connectors cost volume and money, for a use case where both are at a premium; most people would generally rather buy a non-modular phone that gets better battery life and offers better water resistance.

As evidenced by the Note 7, batteries also need a decent bit of structural integrity around them to be safe, and the best way to accomplish that is just a 2-part box that's been glued together. This has the side effect of making water resistance easier and slimming the phone down. Any swappable battery inherently has worse water resistance and volumetric efficiency than a pouch csll in a glued box, and manufacturers value both good battery life and things not blowing up.

Smartphone CPUs (or rather SoCs) contain not only the CPU bit, but important parts of the radio, camera processing, location processing, and GPU. Therefore Android ones come with driver packs from manufacturers that make it all work, but if you're swapping components around you need to make sure your sensors like cameras have a way to install drivers (and a general compatibility standard); which means at best you're swapping pretty big chunks of a smartphone at a time.

Even without forced obsolescence, smartphones have a relatively fixed lifespan of ~5-8 years due to software upgrades, how carriers buy up new radio frequencies, physical damage, battery degradation, vibration motor wear, camera focusing mechanism wear (from vibrations) and in the case of AMOLED display wear. This isn't a knock on standardization, but you can already achieve this lifespan out of a smartphone if you use something like Advanced Charging Controller to limit battery charge to 80%.

If you look at Project Ara (which is the most extreme case of modularity), it failed because of all of the above. The battery life, structural integrity, cost efficiency, compatibility, and waterproofness would all be awful compared to a standard phone.