If I search the internet for "weather balloon" I get much higher prices than if I just look for "big balloon". Just to compare these two listings in particular: this \$320 weather balloon claimed to lift 2kg vs this \$5 big balloon which one can calculate to lift 2.5kg+. Which makes me wonder what the drastic difference in quality must be? Is the former supposed to be much 64 times as durable? But aren't weather balloons supposed to pop when they reach a certain height anyway? And the latter seems to withstand a man sitting on top of it, is that really not durable enough to fly up in the sky?
Short answer: because it is expensive making a large balloon capable of high altitude flight.
Balloon lift is displaced air minus weight of balloon so if you want to do high altitude flight you need something light weight, would suspect that particular $5 balloon is going to be heavy, quite possibly using most of that 2000g nominal lift just supporting itself.
Weather balloons are also intended to expand as they go up to partially overcome the decreasing air density with increasing volume. If your need is just to have an oversize balloon to play with on a string the $5 model would be fine floating around at <50 feet, but if you are trying to actually lift a payload any balloon capable of having someone sit on it would level off at a very low altitude.
The price difference comes back to quality and materials. Weather balloons are normally natural latex to get the stretch needed for the volume increase at low enough weight. This is an expensive and fussy material to work with needing careful control of the raw materials (coming from plants) and humidity during processing. For the $5 version they can use some sort of cheap polymer, and going much thicker any minor variations will matter far less. Having a 0.1mm thinner patch on a 2mm thick balloon is rather less of a problem than 0.1mm less on a 0.2mm balloon.
There may also be some degree of cost because people will pay it. If you are flying a couple of thousand dollars worth of payload spending a couple of hundred to get a reliable balloon is easy to justify.
Would also suggest some creative pricing on that a $320 weather balloon, some random googling for actual weather equipment sellers makes it looks like someone is re-selling with a markup.