Problem is these days the THC content is 5x or more higher than it normally was because strains are selectively being bred for higher THC content.
No different than anything else...if you drank 5x more alcohol or took 5x more acetaminophen or took 5x more Ibuprofen, etc it's going to cause more severe issues that it wouldn't cause at lower doses.
People are using the same amount but at 5x higher strengths but yet don't seem to understand why this would be an issue as if weed somehow is different than every other substance in earth in that regard.