Why does custom officer stop me for carrying too many credit cards?

Immigration and Customs officials are generally on the lookout for anything that could be an indicator that a traveler is either carrying something illegal, or is bringing something in that may be used in the commission of a crime whilst in the country.

Having 16 bank/credit cards is obviously not illegal - presuming that they are actually legitimate cards, and all belong to you. However if they were counterfeit cards (eg, cards made using stolen/skimmed card details) then they are illegal.

Of course, even if you were only carrying a single card it's possible that it could have been counterfeit, but the simple fact is that the more cards you have, the higher the chances they were not obtained legally. I'm sure you'd agree that someone carrying 500 such cards is probably doing something illegal? How about 100? 50? 20? There's no specific number that is deemed "bad", but the higher the number, the higher the chances there is a nefarious intent.

The fact you were searched in the first place also implies they may have had a specific concern about you for some reason. It's possible that it was simply a random search, but these are generally rare - it's more likely they had already seen something else which had flagged you as a passenger of interest, so finding you were carrying so many cards would only add to their existing concerns.

The only way that the customs/immigration staff can address their concerns is to talk to you - which is seemingly what they did. You haven't stated, but presumably they asked you questions around why you have so many cards, when/where did you obtain them, why you are carrying them all, etc. ie, questions that allow them to reduce/remove their concerns that the cards may have been obtained illegally, or will be used to fund illegal activities. The fact they let you proceed means they were successfully able to do that.

I am not sure, as I am not a border official, but I suspect they assumed you had either stolen cards or fake cards.

Most people travel with one or two credit cards next to one or two bank cards and a handful of cards in the same size which have nothing to do with money.

Having so many more is likely something rare and as I have heard many stories of people with a big selection of cards having fake cards or in an other way illegal financial dealings.

Usually banks are reluctant to issue similar cards to the same person, so that would imply active relationships with many different financial institutions, something that is quite unusual, and therefore suspicious.

Personally, I have an unusually large number because I have business and personal and some offer kickbacks and some have advantages on foreign exchange fees under certain conditions etc. Sometimes cards are flagged and stop working or some jetlagged person such as myself leaves them in an ATM so carrying multiple cards offers some security against not being able to complete a transaction. But still not close to 16, nor would I carry all those cards on a trip.

I also think that their suspicions must have been aroused by something prior to that, for them to even know the contents of your wallet- that you were holding so many credit cards. Whatever it was, your cornucopia of cards did not allay their suspicions but likely increased them, so they are going want to delve deeper before they make a decision to admit you or not. It's pretty rare to see border folks rifle through wallets or purses- I've seen it only once (an unfortunate lady ahead of me at US customs pre-clearance).

Border and customs inspections place the burden of proof on the traveler/importer, not upon the government. Thus anything at all unusual can trigger heightened inspection and/or refusal.

Additional cards increases your ability to spend, which could call into question your intention to leave at the appropriate time.