During the planning process this was asked, and I believe they chose not to use one due to the design changes, added cost and work required for water proofing. A baro is pressure sensor and needs a port, ie a hole in the device to sense external pressure.
My understanding is that Stryd doesn’t use one either and are relying on their accelerometers to determine distance over grade.
Garmin likely chose that route because I believe they use a 3-Axis accelerometer where these other devices are 9-Axis.
Edited to add: Garmin is getting not only baro data from the watch, but pace data for the power calculation as well, in addition to Vertical Osc and GCT.
Measuring GCT from the chest or waist isn’t optimal. Anyone who has relied on GPS instant pace understands that this will likely yield some pretty wild power numbers. Not a knock at Garmin, but a reality of GPS in a watch.
runScribe Plus with it’s profile support should be a very accurate source for speed or pace, making the instant pace reading on the watch usable.