[kawfey]

Some do. Edge of Space Sciences (EOSS) is group of citizen high altitude scientists, and their large balloon flights include certified ADS-B transponders [0] and radar corner reflectors. They also file their flights with the FAA to publish NOTAMS. They have significantly larger payloads than this, but are designed to quickly ascend to ~100kft and pop reducing the loiter time in congested airspace.

Project Loon balloons also show up on Flightaware, so they either have ADS-B or TIS-B.

A situation like this will almost certainly cause some congresspeople to fret and write bills that would require ADS-B on all balloons, which would be a death knell for amateur ballooning unless ADS-B (or "legacy" Mode A/C/S) transponders become significantly smaller and more affordable. Mode C/S transponders are already available in miniaturized form factors thanks to the UAS industry, and are designed to be interrogated by aircraft equipped with TCAS (i.e. all 10+ passenger aircraft) that provides pilots deconfliction commands automatically and with no ATC support. But they're still priced for industry, not amateurs.

[0] https://www.eoss.org/ Look for N991SS, N992SS, N461SG.