I don't know which one you are using, but that seems to be the trend in most paid surveys. It is how they keep you hooked, and basically how they make money.
Some paid surveys may allow you to redeem rewards as long as you have enough points or credits in your account, regardless of how many times you have redeemed before. Some paid surveys may limit the number or frequency of redemptions that you can make in a certain period of time, such as per day, week, month, or year.
Some paid surveys may have a fixed or variable number of rewards available for each redemption option, such as gift cards, cash, or products. Some paid surveys may change or update their rewards catalog from time to time, adding new options or removing old ones.