It's been more than 10 years (since I designed hardware with RS-232), and we're still using RS-232?
What do you mean - just 10 years.  RS-232 was defined in 1962 and so is coming up to its 48th birthday.

RS-232 uses a simple asynchronous serial data transfer method that requires very little silicon, is easy to code in software and can be used on micros with minute amounts of RAM or program memory.  It has also had the advantage in the past that all computers and dumb terminals had RS-232 interfaces so using it for debug output was a no-brainer.  

If you needed higher speed or long distance transmission then you just replace the line drivers with RS-422 or RS-485 differential driver/receivers and no change to the software was needed.

Implementing USB on tiny micros is just not possible.  The processing/memory/silicon overhead is just too costly.

SPI or I2C are possible (and used in some places) but the interface cards for your pc are unusual and expensive.  I2C has a nasty lockup state that can make it die if the clock stops halfway through a slave data output cycle and the data bit is low.