Why is peer review so random?
Good question. Hard to answer. Some thoughts:


reviewers are not trained
reviewers are anonymous
reviewers receive minor feedback on their performance
reviewers are also authors, competing for the same funds/ prestige
reviewers are specialized in a narrow discipline
reviewers are volunteers
reviewers are scarce
the review system lacks an external (independent) control system (audit) 
reviewers are humans, with their own personal interests, emotions, capabilities


Considering these observations, it is unlikely to expect two review reports to be aligned. Then the difficult decision transfers to the associate editor who is also a volunteer and not specialized in the author’s field.

Leaves the question why it is accepted while outside science this wouldn’t be. Honestly, I don’t know. Just some guesses:


Science is a powerful isolated sector with its own rules?
The current system works for established research groups?
Jourals do not have the funding to train and attract qualified professionals/scientists as reviewers?
There is no easy solution or alternative?


Added based on comment:
- reviewers are busy scientists
- reviewers are career-wise not rewarded for conducting reviews