Hello!

I'm reading here for a while now, but today I want to come up with a
question that bothered me for a while. I'm not a pilot (but will begin
to take flight lessons soon) so this is mostly out of curiosity.

I know that all the airplanes that people build by themselves fall
into the experimental category, because they are not factory-made,
serialized products but individually built with different quality and
with different modifications. I also know that GA airplanes made of
composites usually fall into the experimental class category, too.
What really annoyed me is that there also are planes that are not
composite and also are factory-made in high numbers that fall into
experiemnat category, like the Aero L-39 Jet airplane. I now wonder
why it's treated as experimental, and not as a normal aircplane,
utility aircraft or aerobatic airplane. Why is this the case? What
makes a jet airplane that is produced by a factory in high numbers
different than say a C-152 that also is produced by a factory in high
numbers? Is the only way to register jet airplanes like the L-39 the
experimental category? Or can they also be registered as say aerobatic
aircraft?

From what I know a pilot who wants to fly a L-39 jet airplane needs
1000hrs of PIC time, and after that needs a Letter of Authorization to
be able to fly the L-39. What if the L-39 would not be registered as
experimental but as normal/utility/aerobatic airplane? Would this also
require 1000hrs of PIC time before someone can fly with this L-39? Are
there any PIC hours required to be allowed to fly turbine airplanes?
Or are the 1000hrs required for experimental airplanes in general?

I also heard that it's not possible to use an experimental plane for
training (PPL, CPL, IFR, whatever). Is that true? If so, registering
airplanes like an L-39 in normal/utility/aerobatic category would
probably also remove some limitations like the use for training,
right? So why are they registered as experimental instead?

As I said, it's just curiosity. But it really confuses me.

JJ