1.) Because I find it morally and ethically repugnant to restrict access to the things one needs to survive for a profit. This is exploitation, and to me is no different than choking someone, depriving them of air, and then demanding a fee to let go of their neck.
2.) Because risk does not entail reward. Some cat burglars are very skilled, and take a great deal of risk when entering a house uninvited to steal someones belongings. This does not in any way confer a “right” to the belongings of others just as the possible risk of loss of investment does not confer ones right to the product of an others labor.
3.) Because labor is the true cost of everything. When you remove all forms of currency and all economic abstractions, the only thing required for production is labor and access to raw materials. Even their precious capital was at the very beginning the product of people’s labor, and while it can be argued that those initial laborers have a “right” to the product of their labor, this right does not confer an infinite entitlement to value produced with that product.
4.) Because nobody has claim to the earth. Private property is an intentionally created social construct designed to centralize wealth and perpetuate class society by granting entitlement to value produced by labor in a legally defined territory. It is justified with the claim that purchase entails a right of increase from ownership, but since there was no original purchase of land there is no legitimate claim.