the voucher is a token from NTUC to appreciate your spending in
NTUC, and encourage you to spend more at NTUC. It is kind of like a
loyalty programme for NTUC to have loyal customers, to keep
purchasing their goods and merchandise.
like what @lemon1974 wrote,
NTUC may not give cash prizes because this may impact itself, as
cash can be used to buy products from its competitors, instead of
NTUC.
Then going by your logic, all NTUC staffs should be forbidden to
shop at Sheng Siong, 7-11, Cold Storage...etc. 
Then why Sg Pools and Tiger Beer can give away cash prizes and
gold bars? By your logic, Sg Pools not scare people take the money
and gamble elsewhere? Tiger Beer not scare customers who won the
gold bars, pawn it and use the cash to buy Heineken?
