Hi
The first serve is a suggested example of an entry point for a scalp trade. In most markets including forex, commodities and a lot of sports events the idea is to buy in a dip. So here you are getting a better price than before the first serve. It is a judgement, because you still have the advantage of the server, but a reduced advantage because the first serve failed, and this is reflected in the price. I am not suggesting back the fav when odds increase, just looking for the “dip” to buy.
I would say the main reason for less liquidity in-play has to be risk. If we take tennis as an example, how would you deal with a game where you have backed the server, and the current score is 0-30 and a break of serve could be looming? The clever people have already taken their money out of the market, because this could be a critical point in the match. Now you cannot trade out for a small loss because there is no lay price available.
On the other hand, pre-off trading carries much less risk, but proportionally smaller profits, and you have to join the enormous queue of money waiting to be matched. This queue of money waiting to be matched is evidence of the lesser risk and why so many people are doing it.
There are massive amounts traded in-play on big tennis matches, but you must be able to handle the risk involved. The risk is not much different from trading football in-play, where everyone knows the direction of the market....until a goal is scored.