Although there is some precision involved in cooperage, it involves fairly few tools, and they are all pretty ancient- plane, saw, wedges, froe, hatchet...even the special router plane that cut the groove in the staves, to hold the ends. The precision is also a matter of fitting- one stave is planed to fit another. And when they're all fit, the barrel is filled with liquid, and the water causes the wood to swell, which effectively closes any leaks. This design to hold water on the inside is also similar to the design of wooden boats of the time- they would keep water on the outside, as long as they were wet. So, for early woodworkers, barrel making is pretty intuitive. But of course people would try other things for holding liquids- like amphorae; but compared to barrels amphorae could only be so big, and could crack, and earthenware is of itself quite heavy.