What is the reason packages are distributed separately?
Are the "single distributions" of Python to come pre-loaded?
If it's part of design to keep the 'core' separate from additional functionality, still in that case it should at least come 'pre-imported' as soon as you start Python.
Where can I find such distributions if they exist?
Many of these tools, including core Python, are separately developed and distributed by different team, so it is up to aggregators to curate them and put them into a single distribution. Here are some notable examples: