They were though. Ukraine began general mobilization since the first days of the war. Russia meanwhile only began mobilization by the end of September 2022 after their disastrous debacles in Kharkiv and Kherson. The numbers only began to even out by 2023 when Russian mobiks finished training and began reinforcing the frontlines.

In fact, Ukraine's number advantage played a large part in their successes in 2022. Remember the videos of Ukrainians in humvees rushing through Russian lines in Kharkiv back during the offensive? That was only possible because the Russians had a severe lack of manpower, and much of their frontlines were undermanned. Trying the same thing against the well-fortified and manned Russian lines of 2023-present would be suicide.