(boring) why is KEEPKERNELS unset and pobj gets cleaned?

Hi list
I marked this thread as "(boring)" so to not anger snowflakes with
boring/stupid questions.

So this thread's question is: I found out that when recompiling the base
system I think that it starts from scratch even though there should be object
files to prevent unnecessary compiling (I did also run "make obj" for the 2nd
time, maybe that broke something), so why is that? (assuming I didn't fuck up
something again)

I'm assuming the process will take another too many hours, and experimenting
with different settings might be too difficult, so that's why I'm here now.

My questions are why that's like that, and if it's better to just recompile
everything again (but how the fuck would you guys get any development done
then? lol)

After reading a few manual pages, by which I mean searching for stuff like
"clean", I stumbled upon the KEEPKERNELS additional variable which after being
enabled, kernel object directories should not be cleaned during "make build",

I'm assuming that this is what I need, so I enabled it in /etc/mk.conf as
explained in the mk.conf(5)#DESCRIPTION section, and I have indeed verified
that the variable was set with `make -p build`.

But then with the power of flex tape(joke) I mean du, I figured out that it
recompiles /usr/obj/lib anyways, and maybe some other directories too?

P.S. I'm following release(8) if it wasn't clear..