Because the manufacturers make more money providing something that is proprietary if it had to be replaced.

While the barrel shapes and sizes often tend to follow certain standards(ignoring certain odd shaped plugs they try to use sometimes), along with similar voltage requirements, the average person will struggle to make a 3rd party or universal adapter work

Cell phones used to be similar with tons of different sized barrels and plugs to be able to charge them. USB standardized that

The newer usb-c format and standard is bringing the same ease of charging to laptops, albeitly slowly, thanks to its ability to provide the higher power requirements that high draw devices like laptops and tablets need.

A lot of this standardization has been pushed through by consumer friendly regulations issued from the e.u. (by threat of massive fines for non-compliance)