Mechanical simplicity and standardisation.
The windscreen wiper is very likely to be outsourced to a 3rd party company, and they can reduce costs by having a repeatable design. In terms of a blade going on a straight line, it’s much easier to rotate a wiper blade from a motor than translate that into a linear movement.
It’s worth nothing that other systems exist (https://www.a1-windscreens.co.uk/news/windscreen-wiper-movem...), but much less common.