As I posted earlier, it is simply survival instinct. As a prey animal you stay with the herd in times of danger. Continuing to run is a sign of stress not enjoyment.
The same could be said about many cruel " sports" - fox hunting, bear baiting, hare coursing etc . Thankfully the civilised majority has recognised the immorality in these activities and taken action to try and end them.
If it weren't for the money involved then undoubtedly the same could be...
The simple fact that, as you say, the design of the whip has changed and and the use of these has been legislated on only goes to acknowledge that their use is cruel.
On the face of it flat racing should be much safer for the horse than races over obstacles. Take a look however at the link I...
If there were co-drivers "encouraging" the driver to go faster by hitting them with a whip then I'm sure there would be vociferous calls for motor racing to be banned.
Not only that they are often injured whilst being hit with a whip.
How anyone can support racing, (the only place you can see an animal being repeatedly hit with impunity).and still have a clear conscience is beyond me.
https://www.horsedeathwatch.com/index.php