These businesses have a natural limit to their growth due to the estate tax. If the company grows too large, the taxes on death can be crippling for families that can not plan decades in advance. Public corporations do not have to pay this tax, and therefore have an unnatural advantage in holding valuable assets over long periods of time. The estate tax is well-intentioned, but because it hits only family firms it’s essentially regressive despite being applied only to wealthy families.
The owners of public companies are irrelevant to the large public company’s continued existence. Owners for them come and go and pay their own taxes, but the company never has to pay the estate taxes itself, hence the advantage against those who must.