We'd play in that amount of rain in my grade matches.
look, the real reason they won't go on even in a little bit of rain is because it then sets a precedent. The players will need to cope with a wet field, wet pitch, wet ball, and then if the umpires don't come back on the next time it's raining they will moan and complain about how it's not fair, and how they did it last time, and they should do it now.
Once you agree then to resume play every time its raining, then people begin to argue about how much rain is okay to play in. No one is accurately measuring how much it's raining in any way that can be used to set a line in the sand of how much rain is too much. It becomes subjective. Plus, the level of rain varies over the course of the shower. It may start as a drizzle but get heavy. It may start heavy, but then slow down. You are now creating more grey areas and more doubt within which players, team managers, and casual internet fans on forums can bitch and moan about how the umpires are crap and how one team is being robbed by being forced in play in wet weather while the other isn't, or how it isn't fair one side gets denied time to take wickets/chase runs but the other doesn't, and it's just a big mess that isn't worth getting into.
So if it's raining, there is no cricket
Plus the whole safety aspect - fast bowlers and fielders running hard on slippery surfaces. Plus the ball gets wet easily and gets ruined. When the stakes are high you absolutely cannot allow these to be factors.
But the answer I want to say is that this is further evidence that cricket administrators care more about your grade cricket than the Ashes tbh