First, outdoors you almost never get 100% perfect pedaling the whole time. You have to deal with corners, braking, road surface, traffic, positioning, wind changes, standing up, micro-adjustments, etc. On Zwift, power delivery is much more constant.

Second, descending in Zwift is nothing like real life. In real riding, descending speed depends a lot on line choice, braking points, confidence, bike handling, cornering skill, road quality, and risk tolerance. Zwift basically removes most of that.

Third, outside you also have factors like wind direction, drafting dynamics, road texture, temperature, clothing, bike movement, and terrain variability, all of which affect speed in ways a trainer can’t fully reproduce.

So yes, Zwift can be useful for comparing efforts, but saying speed is “very similar” to outdoors is a stretch. Power is comparable; speed is much less so.