entering a country and going through customs check?

When you use banks or money sending services transactions over a certain amount are reported to the authorities as are big numbers of smaller transactions if caught.

That is not what criminals want, so they try to sneak in cash.

Some of the cash carrying people just want to avoid paying tax, most want to avoid ending up in prison for illegal activities.

The problem wasn't the amount of money but not reporting. You can do electronic transfers, but it still needs to be reported if it exceeds the maximum you can bring into the country without reporting. For the US, you need to declare amounts over $10,000.

yes ofcourse, depending on the juristriction, most countries run a rule of 10,000, euro pounds dollars or equivalent per person as a max amount of money in cash. Otherwise you should wire it electronically or just use your own bank account.

Carrying large amounts of cash is unnecessary these days as most accounts will operate internationally and all you need is a debit card and credit card. Having large amounts of cash can be an indicator of money laundering.