Can someone explain the RIDICULOUS spending targets of Japanese Credit Cards?

I collect cashback/points and the like on credit cards as a hobby. Used to do it in the UK and was hoping to do so in Japan but something that catches my eye is the huge discrepancy of what card issuers ask new customers to spend. This is noting that most essentials in Japan cost half that of the UK (and salaries roughly are half too).

An example, the Amex Platinum. UK / US / JP. Let's compare the new customer spending target (aka get bonus for spending X in Y months).

• UK: £6000 in 3 months (1.1m~ yen / $8k~)
• US: $8000 in 6 months (1.1m~ yen / £6k~)
• JP: ¥4,000,000 in 6 months (£21k~ / $28k~)

Not to say I want an AMEX Platinum but this paints a similar picture I've found with many other Japanese CCs (such as the JAL Club-A Gold, 1.2m 2 mths). Can anyone explain why? The US/UK targets are difficult to hit already, what's the aim here?