You need to put more than one LED in to replace the old bulb.
That is true of many LEDs (especially the cheaper ones), but not of all. It depends on how bright the LED in question is (which is controlled by both the LED specs, and how you are driving that LED). And while many of the cheaper LEDs use an older technology that isn't as bright (requiring you to use multiple LEDs for higher lighting conditions), some of the newer LEDs are bright enough to use a single LED as a bulb replacement (for pretty much all car bulbs EXCEPT for the headlights).
As an example of some of the brighter LEDs, some "Luxeon" LEDs (when combined with a proper driver circuit for the LED) are very bright. Not only are my tail/brake lights SINGLE (Luxeon) LED modules (I took the "easy approach" of buying premade "bulb replacement" modules that already had the LED and driver circuit built in), but those single LED modules are BRIGHTER than the stock bulbs they replace. And I even use single LED modules as replacements for my "backup lights" (yes, a properly built "high end" single LED module really is that bright).
So I know it can be done with the proper LEDs. Of course, if someone just buys the cheap LEDs in bulk, than multiple LEDs will have to be used to get enough light. But with the proper LEDs, it really is quite possible to get more than enough light for a tail light, using only a single LED).