Why is nuclear+short term storage not an option
Because cold winter days exist. Yes you can only build nuclear capacities for the average day and then short-term storage to match the demand pattern. But you would need to do so for the day(s) of the year with the highest energy demand, some cold winter work day. What do you do with those capacities the remaining year as throttling nuclear down is not really saving much costs (most lie in construction and deconstruction)?
130% production on average, with excess being stored, minus losses in conversions, transport and storage = 100% demand covered all the time.
Or the longer version: For a stable grid I need to cover 100% of the demand in next to real-time. This can be achieved with enough long- and short-term storage, plus some overproduction to account for storage losses. The 115% to 130% production (compared to actual demand) are based on studies for Germany and vary by scenario, with the higher number for the worst case (people strongly resisting all changes to better balance consumption and south Germany keeping up there resistence to diversify by only building solar while blocking wind power).
The question now is: How much storage do I need? And that answer is varying by much greater amount based on scenario (for example between 50 and 120 GW capacity needed as electrolysis for long term-storage or battery storage between 50GWh and 200GWh).