That's because there is so much within walking distance. If you can comfortably walk a mile or two at a time, you can walk to virtually all of the major museums and tourist attractions in central Amsterdam.
And much of what you may want to see and do may be in a much smaller area than that.
And if you get tired, there's always a cafe, bar or coffeeshop handy where you can stop and rest. There are also numerous little parks and plazas with shaded benches where you can just stop and relax.
Of course, in the coldest months of winter you may want to rely more on trams or other transport, but when the weather is pleasant, walking is definitely the way to go. Besides, that it helps use up some of those calories you'll be ingesting from all of that delicious Dutch pastry and cheese and all that great Dutch and Belgian beer.

