Long-time seller/distributor here -- the main reason for this is mandated by brands, who want to make sure their MAP (minimum advertised price) is respected across all channels.
Basically different distribution channels (speciality shops, big box marketplaces, and ecom stores) have very different levels of overhead, so if each channel was allowed to set their own price, you'd end up with brick and mortar stores doing a lot of showrooming and then online stores gaining the bulk of sales because they're cheaper (because their overhead is low).
This pretty much happened in the early 2000s-2010s so over time brands became VERY particular about enforcing MAP.
Basically different distribution channels (speciality shops, big box marketplaces, and ecom stores) have very different levels of overhead, so if each channel was allowed to set their own price, you'd end up with brick and mortar stores doing a lot of showrooming and then online stores gaining the bulk of sales because they're cheaper (because their overhead is low).
This pretty much happened in the early 2000s-2010s so over time brands became VERY particular about enforcing MAP.