– because I have no time
– because whenever I have a good idea I’m in a train and I don’t like people looking over my shoulder
– because I don’t have my favorite pen
– because I don’t know in what language to write
– because I could be doing something immediately useful instead
– because my handwriting is ugly and when would I possibly type this over?
– because characters of my stories have gone quiet on me
– because I never finish stories
– because all I think of is bullet points
– because I have an awesome book to read / podcast to listen to and they tell stories way better than I’ll ever do
– because the constant pings in my inbox and social media have reduced my attention span to that of a hummingbird’s
– because all the good stories have been told
– because I’ll write tomorrow
Smithereens 
I think I’ve used all of these and more…
I can add a few:
– because I am tired right now
– because although I know more or less what I want to say I don’t know how to get started
– someone else has already said it better
About the bullet points– writing reports and PowerPoint presentations deflects the wide-ranging imagination you need for fiction.
Here’s another one – I can’t decide which idea to work on.
Here’s one of my favorites: because I have nothing to say
I’m glad you recognize these as all nonsense. I just saw something funny on a friend’s tubmlr – it said “If you are worried that your writing isn’t good enough – stop. Temperatures are rising and soon all writing – good and bad – will be under water.
LOL. I can certainly relate to all of these. Also, “Because I’m the only one who would be at all interested in reading that, so why bother?”