To make something modular you need to wrap each piece in its own case, add bulky connectors, etc. Both weight and volume are at an extreme premium in mobile devices. All that metal/plastic going in to making the pieces modular is stealing volume and weight from the battery.
The only realistic way to make the power envelope is to use an SoC, which means the CPU, GPU, and RAM must all be in the same module. That doesn't leave a lot worth upgrading... maybe just the radio module. Jumping up in screen resolution would mean replacing the SoC to get a better GPU too.
Modularity worked in desktop PCs because they have gobs of space and an AC power connection.