Why do many serious systems have their monoblocks mounted on the floor between their speakers.
Because it's a convenient place to put them for those that choose not to use a rack or table and there is a possible sound benefit.
It would seem to me they could be quite susceptible to the occasional kick or spilled drink.
They are on amp stands, so no worries about them getting kicked and if you think the owner is getting anywhere near them with a drink in hand, you're dreaming.
Is the sound improvement (if any) worth the risk?
By not having a structure between the speakers, which could interfere with the sound stage, there is a possible benefit, but depends on the structure, the speakers and the room. As for the notion of short cable runs being a benefit, nope just a myth. If you look closely at that photo you will see the speaker cables are snaked around and are likely 8 foot cables at a minimum.
