Because it was a terrible episode that they didn't think through at all.
After Where No Man Has Gone Before warp was always scaled up toward warp 10 as if it was an unbreakable barrier, and technical manuals released well before Threshold said warp 10 was infinite speed. The problem with the episode is more that it's total garbage than it's use of warp 10 as a hard barrier.
To be fair it's not a barrier more so than something way off in the distance not worth thinking about.
The speed of light is figureouttable. 186,282 miles per second thereabouts, but I don't think warp ten is?
10,000 times faster than warp 9 is still not warp ten.
You just keep going faster and faster until you turn into god.