Because it is surveillance. There’s plenty of surveillance examples with multiple uses and “improving the service” doesn’t blanketedly discount their other uses.

More importantly they make it a pain in the ass to copy the actual URL of a link without actually clicking it. If you right click a search result link then their JS edits the href to the Google tracking link. So you can’t actually examine the entire URL without risking opening it up and being tracked. At best you get whatever preview your browser shows on hover.