why, when i buy a ratchet in germany,is it called a 3/8" drive
ratchet, when they are on the metric system, the sockets are all in
metric, but the drive is a 3/8" square drive size, why is it not
called perhaps 10 mm??

i have seen it in german tool catalogs called by the 3/8" designation also.

In certain areas, where metric conversion was never necessary, the old
inch ("Zoll" in German) is still in isolated use. For example,
plumbing items (pipes, valves, joints etc.) are usually referred to by
their old inch designations. However, all tools and items are defined
in metric units. The use of pre-metric units is just an old tradition
followed by certain professionals. A German master plumber would
hardly say "A 2.56 cm pipe", he'd say "Ein zölliges Rohr" (A one-inch
pipe). But he would not use the otherwise irrational old unit for
anything else.

It's just tradition and habit, nothing else.

Scriptor

People all over the world use square-drive sockets of size 1/4, 3/8,
1/2, 3/4, 1...  inch.  So while many countries use metric nuts and
bolts, the metric sockets used to drive them still have SAE sized
drivers.  That way sockets and drivers made in different places by
different companies are compatible.

http://www.sizes.com/tools/socket_wrenches_metric.htm

Zee Girmans vill doo vatevah zee vak zay vant!

vak ?
Que es vak?

The Germans will do whatever the <ahem> they want?

borrowing imperial(or metric) terminolgy is something that just
happens.  35mm film is common.  The tripod socket in the base of all
cameras is 1/4 Whitworth, even thought the rest is metric.
 
 Same as Americans buying a length of 2 x 4 timber.  We buy a length
of 4 x 2. It simply happens.