Size, for one. You'd have to build a trigger that includes a MIDI circuit board (pictured). And it would have to be powered. And then it still needs to talk to something. Your MIDI trigger signal has to be sent to a sound module that has a MIDI input. And that sound module, then, has to be hooked up to an amplifier.
This really over complicates what could be/is done with a standard drum trigger ..... hooked up to a drum module.
Size - That seems to be some kind of micro (arduino?) dev board bristling with I/O pins and a USB port that are spurious to the application. A properly engineered solution could be much smaller than that. It could be a single chip solution that could easily fit into the housing of some of the existing drum triggers. This is certainly not a point of contention. MIDI has 5V for power, albeit the current available is small, however the micro or ASIC in question wouldn't need much power.
Yes, it needs to be hooked up to something that utilises midi messages. In my case, this is a laptop that is already on stage that has a sampler running along with other VSTs.
It doesn't overcomplicate what is done with a standard drum trigger hooked up to a drum module. It is a different device for a different situation.
Standard drum triggers are there for people who want to use drum modules. I am not suggesting that this device replace either of those things for those people.
I want to use a standalone VST sampler without the added bulk and complexity of a drum brain and its power supply solely for the purposes of interpreting the drum trigger signal into midi messages for use in an external sampler.