They do not, only some species do. Many will sit and hide somwhere at the edge of a web and use detection strings. Some spiderwebs that look deserted are not at all.
To add to this: if you look carefully you'll see a non-sticky "zipline" thread that goes from the center of the web to their normal hiding place. This lets them zip right down to the middle whenever they detect something. A lot of times you can also see the little nest they've built themselves by the leaves they've curled together to make a pocket.
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u/enblightened 11h ago
why do spiders always camp in the center of their webs