The newer Switch models use a smaller node for the APU. Nintendo had a lot of available overhead but they chose to keep the system down-clocked and more than double battery life. A long battery life is more important to a handheld console than higher resolution.
Even with the downclocking the newer systems actually have better performance than the original models.
PS4 Pro doesn’t have to worry about battery life. The Switch need to limit itself to 15 watts or less to avoid overheating and battery life would be so short that it would be useless as a handheld console.