One reason a collision hasn’t happened is the launches are coordinated with local air traffic control. Here’s an article on the subject.
http://weatherjackwilliams.com/answers-weather-balloons-and-airplanes/
Those are National Weather Service balloons. They are launch every day at 0000Z and 1200Z GMT.
The FAA regs under FAR 101 (14 CFR Part 101; covers moored balloons, kites, unmanned rockets, and unmanned balloons) state that you do not need a waiver from the FAA to fly a balloon as long as abides by these weight rules:
(4) Except as provided for in 101.7, any unmanned free balloon
that-
(i) Carries a payload package that weighs more than four pounds and
has a weight/size ratio of more than three ounces per square inch
on any surface of the package, determined by dividing the total
weight in ounces of the payload package by the area in square
inches of its smallest surface;
(ii) Carries a payload package that weighs more than six pounds;
(iii) Carries a payload, of two or more packages, that weighs more
than 12 pounds; or
The payload is only 11 grams or 0.0242508 lbs.