Because there are rules, bending the rules based on the situation is what happened in Abu Dhabi and it made the sport a laughing stock.
Max didn't "lose grip", he was pressing to come out ahead of the Ferraris and came across the track to close the door and gained a massive advantage. You can't speed in the pitlane and claim "oh, it was wet and I couldn't brake in time", the rules were changed this year from "touching the line" to having an entire tyre over the line, but the RD sent out notes with the old rules and that's why it was protested. If the rules hadn't been changed he should have received a penalty. You don't get to claim "I broke a rule, but didn't get an advantage so we're good right?" or "I broke a rule, but its wet so all good, right?". The Stewards will take weather into account in situations of collisions etc., but not in the case of crossing lines, speeding, cutting corners etc.