Why is it so difficult to finish things?

I wonder about this. I often have a burst of inspiration and do great work. Also more often than not that burst is just that. A burst as per the Google definition of it:

suddenly begin doing or producing something.
“Sophie burst out laughing”
synonyms: break out in, launch into, erupt in, have a fit of; suddenly start
“she burst into tears”
exclaim, blurt out, ejaculate, cry out, call out, shout, yell
“‘Well, I don’t care!’ she burst out angrily”
suddenly start
“he burst out crying”

“suddenly begin doing or producing something” The problem that i am experiencing is that it is short lived.

That last part of the quoted definition resonates with me when I think of this.

I really do not have the answer for this question. I am however grateful for the few occasions that the stars align and this burst sticks around long enough.