The LEDs are vastly cheaper if you buy them in a strip. Like $10 will only buy a few loose LEDs but it will buy a strip with a hundred of them. Somehow there's a big premium for being able to solder them yourself.
For 'street lighting' you can probably run thousands of these LEDs off one pin. Each LED cleans up the signal for the following ones, so there's no degradation over long runs. With some of the faster Arduino variants such as the Teensy 3.2 you can even do full-motion video on thousands of LEDs.
The real limitation is the Arduino's memory space.