Technical difficulties of development and maintenance of own platforms have been mentioned in other comments.
However, many own platforms lack revenue opportunities provided by centralized platforms. YouTube specifically has a huge benefit of built-in monetization. Most content creators on YouTube start earning money much earlier because YouTube manages ads for them. General trend I see is creators start getting sponsored videos sometime between 500,000 and 1MM subscribers. Depending on channel that can take about a year getting videos out at a regular pace. I hazard a guess that many would’ve given up much earlier if they had to think about monetization on their own instead of relying on the platform for that.
Technical difficulties of development and maintenance of own platforms have been mentioned in other comments.
However, many own platforms lack revenue opportunities provided by centralized platforms. YouTube specifically has a huge benefit of built-in monetization. Most content creators on YouTube start earning money much earlier because YouTube manages ads for them. General trend I see is creators start getting sponsored videos sometime between 500,000 and 1MM subscribers. Depending on channel that can take about a year getting videos out at a regular pace. I hazard a guess that many would’ve given up much earlier if they had to think about monetization on their own instead of relying on the platform for that.