Can you please explain to me what's happening here?

[Moniac]

why there is this kind of useless situation about the WOM in tennis, and not in horse race?

This is the answer.

Weight of money only applies to pre-off racing / markets that are driven by pure opinion, it has effect on an inplay sport as that is reliant on the underlying metrics of that sport. In Tennis, the score line.

[Emmson]

WOM is still a consideration in cricket markets that have gone in-play especially test matches.

[Derek27]

why there is this kind of useless situation about the WOM in tennis, and not in horse race?

This is the answer.

Weight of money only applies to pre-off racing / markets that are driven by pure opinion, it has effect on an inplay sport as that is reliant on the underlying metrics of that sport. In Tennis, the score line.

Euler's quote should say "no" effect!