Why do older cars have high horsepowers but low top speeds? ?

High speed is more a result of shape than of horsepower..  For many cars it takes a LOT more horsepower to go a little faster.
My brother had a 427 cu in Camaro . It could spin its tires at highway speed, but at 130 mph it wanted to fly and he could hardly hold it on the road.  Twice the horsepower would not have made it handle any better at that speed.

what cars? most cars that are old actually had low horsepower and high speeds! mclaren f1 623hp 241mph, saleen s7 turbo 723hp 247mph, not sure what cars you mean?

My 1954 220 Mercedes Benz had a top speed of
130 mph

They were also heavier.

I saw a 1970 Dodge T/A Challenger go 150 MPH across the San Mateo bridge. That's what was done back then with 290 HP.

They had some pretty fearsome top speeds too. With tall gears a lot of those 60s cars could approach 200 mph at Bonneville.  Then there is how you measure horsepower. There are a lot of discrepancies. I remember working in a motorcycle shop when a pissed off customer asked the owner why my 30 HP Husky was so much faster than his 36HP Yamaha.   The owner replied that my Swedish draft horses were a lot bigger than his Japanese ponies.

It was all about getting up to highway speed quickly. Not racing at breakneck speeds.

The manufactures designed the cars to meet specific standards of operation.Mostly to save on fuel expense.

Because car designers gave a dam more then, plain and simple!!!!!

less stress on the motor and better fuel economy and less wear on the motor .