A VLEO satellite orbits the earth in under two hours, giving it visual coverage of the planet along the satellite ground track for a dozen or more orbits per day. Launching enough balloons to get that kind of coverage, especially without the ability to control their position would cost an astronomical amount. You're not going to get anywhere near the same coverage and they'll probably all end up blown by the same wind system into a vortex and get stuck there anyway.
Weather balloons are inexpensive because the radiosonde payload is very cheap and light, not requiring much power or other infrastructure. Putting a proper surveillance payload on it would dramatically increase the price mostly because it'd then have to power the payload. That role is currently mostly done by UAVs.
Weather balloons are inexpensive because the radiosonde payload is very cheap and light, not requiring much power or other infrastructure. Putting a proper surveillance payload on it would dramatically increase the price mostly because it'd then have to power the payload. That role is currently mostly done by UAVs.