The speed the rain comes and goes is much quicker than in real life, not sure if 10 or 15 times is exagerated or not. Also for the tyre wear and fuel multipliers they're often up around 6 or 7 times real speed.
So for say a 10 lap race with a 6* multiplier, that means the tyre strategy should effectively be the same as for an actual 60 lap race.
Imagine a choice of hards for 60 laps, or softs for two 30 lap stints. If the soft strategy is worth on average 0.5s per lap, that gives 30s over the whole race. About right to justify a pit stop. But with GT7's accelerated time you only actually gain 5s over the whole race. So no way is the stop justified.
What they should do is divide the time lost in the pits by the in race multipliers so in this case a 30s pit stop becomes a 5s stop then the 1 stop strategy becomes viable.