The reason that most people can't braid their arm hair is
simply because it grows for a shorter amount of time than
head hair. This may seem painfully obvious, but there's
still plenty of cool science involved.

Hair length is genetically determined and varies from
person to person. The growth cycle of hair is known as
the "anagen" and arm hair's anagen only lasts a few weeks
before it starts falling out.

Head hair, on the other hand, can grow for years and years
before sliding down your shower drain. That's not to say
that all your head hair is constantly growing. At any given
moment only 80 to 85 percent is growing, while the rest
waits patiently to fall out. If you're keeping a log of
trivial science words, that waiting phase is called
"telegen".