It's the same reason why the macbook pro tops out at 16 GB, most laptops use U series CPU's and optimize for thinness.
The U series CPU's are typically matched with (soldered) LPDDR3 for battery life and thinness, which maxes out at 16 GB. The Xeon and H series CPU's are matched with DDR4 for performance and therefore can go past 16 GB. AFAIK the U series CPU's with 32 GB will come with the cannon lake generation with LPDDR4 support, which should start shipping this year.
Lenovo used to sell the T470p which marries an H series CPU with a small (but not thin) form factor, but since the 8th gen U series chips now also have 4 cores / 8 threads (basically matching the 7th gen H series for performance) they've silently dropped that line, opting for a simpler line-up based on U series across the board, topping out at 16 GB in many cases.
The T470 and the T480 both support 32GB just fine. They didn't drop a thing especially because the T470 and the T480 is the exact bloody same aside from generational IC swaps but it's the same chassis and the same planar. So yeah both have the same two SO-DIMM slots. The T480s is an entirely different matter. Finally, the T470p has a very limited life because it doesn't have Thunderbolt 3. The ignorance in this thread is painful.
The 8th gen U series is not "basically matching" the 7th gen H series, the base clock (i.e. what you actually get under load) of the i7-7700HQ is 47% higher than the i7-8650U.
I have a 32gb t460p (skylake) myself and it is still an awesome machine. All day battery life during normal web dev with the extended battery pack, but it can also handle a 20 gb ram hadoop vm without bogging down, and bioshock infinite ran nicely on the nvidia graphics. It has just one real compromise: it looks very dated.
It seems they intend the t480 series to be its successor, now that you can get quad core cpu’s with dedicated graphics in that line.
The U series CPU's are typically matched with (soldered) LPDDR3 for battery life and thinness, which maxes out at 16 GB. The Xeon and H series CPU's are matched with DDR4 for performance and therefore can go past 16 GB. AFAIK the U series CPU's with 32 GB will come with the cannon lake generation with LPDDR4 support, which should start shipping this year.
Lenovo used to sell the T470p which marries an H series CPU with a small (but not thin) form factor, but since the 8th gen U series chips now also have 4 cores / 8 threads (basically matching the 7th gen H series for performance) they've silently dropped that line, opting for a simpler line-up based on U series across the board, topping out at 16 GB in many cases.