Let me explain. The robbers are not actually targeting the metal gates, they are actually targeting the items within for example your items that were bought in preparation for reno, there was a case of someone apartment's electrical circuit board and switches being stolen.
Though there is a wooden door, during reno, the contractors only chain/lock the main gate while leaving the main door unlock. (pls do not give your main gate and door key to contractors, heard cases of the keys being duplicated)
This problem of the metal gates not being properly secured is a old one, and it happened to my previous flat 30 years back.
For those interested to see the workmanship, you can go 220b 8th floor lar, everyone that have gotten the keys have done it lol.