Netflix provide discounts. If you are a new customer, they often give you a month of the service of a higher tier for the price of a lower tier when you sign up.
Amazon also provide discounts on Amazon Day and Black Friday.
BritBox also provide discounts.
Only major streaming service with no discounts as such are Disney+, but as I took the annual £59 deal back in February, they may have a retention offer next February.
I am comparing like with like. The full prices of the services with each other.
I have never, ever had an offer of a discount other than my original sign up from Netflix. Amazon discounts are for Prime membership, not for Prime Video subscribers.