If you break down the math, the orc has a proficiency bonus included in its attack. Almost every monster with a weapon in its stat block is proficient with the weapon (though there are, interestingly, a few monsters who aren't proficient with their own natural weapons). Much the same goes for armor: you don't see any nonproficiency penalties in the stats for scale mail, so the orc is proficient.
More broadly, you have to remember that the stat block is intended for brevity and ease of use and at no point claims to be exhaustive, so in more complicated situations you have to again go beyond the stat block and decide what's reasonable. The orc also doesn't have a thrown rock attack listed, but it's an ordinary humanoid with ordinary humanoid capabilities, so presumably it can do that. It's also a warrior-type character, so if it picks up a warhammer or flail or other martial weapon, it's probably proficient in that too.