I think the main reason the sellers are not selling is because everybody who wanted to sell at this price would have done so a long time ago by now.

And yes there are certainly a lot of other reasons too like the tax issue you mentioned. I myself have never sold anything partly also because the taxes make it confusing and complex

Especially when you have millions/billions worth of crypto this is a bigger problem. There is no way to cash out all at once without giving away half of it to the taxes and such. This is why I think Monero has for a long time been a big thing amongst whales in which they can put their profits, and why it is certainly undervalued for this reason by the wider crypto community. But the value of it is only clear once you run into these problems