Why is undercutting allowed when your opponent has a higher number of cards left than you do?

Answer 1

I assume you are talking about Rummy or Gin Rummy. Here is why
The object of Rummy is to get a 4 of a kind and a 3 of a kind (or a 4 str8flush or a 3 str8flush). In Gin Rummy you are supposed to get a 4 of a kind, and 2 3 of a kinds (or str8flushes)
If you make Gin, you get a 25 pt bonus and cannot be underknocked
If you do not make gin, you have the option to knock but it is risky for 3 reasons.
1 - You do not get a 25 pt bonus
2 - Your opponent can add his cards onto your melds
3 - After doing that the opponent flips up his melds
4 - any unmelded cards are how many pts he gives you, but you need to subtract how many pts you knocked with from it. So if you knock with 2 and he has 5 points, you get 3 points (5-2).
5 - if your oppenent points is less than or equal to what you knocked with, he underknocks you and gets 25 pts plus the difference of the underknock

So the best advice i can give you is knock with a low card if it is early, and go for rummy or gin if it is deeper in the game