We "did." [0]
What we did not know was an accurate way to determine longitude, specifically on a ship, until 1761. [1] [2]
Consequently, any voyage before 1761 knew its latitude exactly, but dead-reckoned its longitude.
33 days of speed-estimated dead reckoning in 1492, plus having no idea of the speed or orientation of the underlying current you're in, leaves a lot of room for error.
[0] For values of "did" that include "the correct answer had been derived and was documented (Eratosthenes, within ~2.5% in ~240 BC, working at the Library of Alexandria), but it wasn't broadly accepted as the correct answer." Thus leading to Columbus believing an incorrect value instead https://en.m.wikipedia.org/wiki/Earth%27s_circumference#Colu...
[1] https://en.m.wikipedia.org/wiki/John_Harrison#H4
[2] Except via some very complicated planetary transits that were pre-calculated and could only be used as they occured. https://en.m.wikipedia.org/wiki/History_of_longitude#Satelli...