As to why they're not "freely" interchanged, well they
are, to a point.
As you study particle physics, quantum physics, chemistry,
et. al. you learn that there are specific possible reactions
or transitions, emissions, absorbtions, conversions,
et. al. that can occur.
There are also specific states or events that can be
observed to exist, for instance, there's a possibility that
an electron positron pair can materialize from the
vacuum field and exist for a while, then come together
and annihilate into high energy photons. Energy
shifts from field energy to mass energy back to field energy.
There can be assigned specific probabilities that such
conversions/events/transitions will occur depending on
the involved energies, momenta, fields, temperature,
et. al.
Sometimes such events are highly probably and occur
frequently, such as the explosion of a firecracker in a
fire, or the melting of an ice cube on a hot day.
Other times such events are improbable to occur at
low temperatures / energies, such as the break-up of an
iron nucleus into individual nucleons, or a piece of rock
catching fire on a cold day spontaneously. It's *possible*,
but very unlikely without a lot of added energy to help
the process occur.
It's like the reaction probability and rate and equilibrium
in chemistry, and also it's similar to the consideration
of the activation energy of a process -- without the right
energy input, even a process that will release a lot of
energy may be unlikely to occur. Thus the stored energy
in a firecracker takes added heat/energy in the form
of lighting the fuse before it'll become released with any
likelihood.
So you can say that the probability of a system *not*
undergoing a given conversion between mass/energy
is a measure of its stability.
In just a few minutes free neutrons will probably
'spontaneously' convert (decay) into a proton, electron,
and added energy. Whereas a neutron inside a nucleus
is usually quite stable and will probably stay there without
decay for a long time in most (non-radioactive) nuclei.
In contrast a free proton at normal energies will
probably not decay or change into anything else even if
you waited billions of years.
If bound neutrons or protons were *not* so stable, we'd
not be here discussing this topic now, because billions of
years ago all the normal matter that makes up our
bodies / planet / solar system / galaxy would have decayed
into other kinds of matter/energy and we'd never have
existed.
So the reason the mass and energy doesn't seem so
freely interchangable is that the universe we're familiar
with in ordinary conditions is a 'cold' fairly low-energy
place where in ordinary circumstances on Earth matter
is stable in the chemical and atomic states it exists in,
and large scale changes of energy/mass do not occur due
to the stability of matter at low temperatures/energies.
Were that not the case, the planet would've "burned up"
and we (being fragile creatures that can live only in
very delicately balanced environmental conditions)
wouldn't exist.
