there’s always a significant additional bureaucratic cost when selling a house and buying another one.

This really only affects landlords and estate agents. Most people looking for a home are looking for a place to stay for life, and any "bureaucratic cost", if you're purely talking about red tape, form-filling, phone calls etc, is more than worth it for a lifetime home. Again, citation needed. If you're talking about a literal monetary cost... whoa, look at that - capitalism!

renting has at least a single clear benefit beyond just being able to afford it: greater flexibility

"Flexibility" is a daft measure, only useful for people who plan to move often, which, again, is not common, except in the case of people needing to move often for work, which - hey, it's capitalism again!

Also, the financial risk is almost zero when you rent.

"Almost" is doing a lot of work in this sentence. The risk of being made homeless by your landlord for petty reasons is a pretty clear risk. Having your rent hiked is a financial risk. Having to bite the bullet and choose an expensive place to rent because it's the only one reasonably close to work is a financial risk. Being under someone's thumb to provide them income is itself an inherent financial risk.

And by the way - what do you think causes the financial risk of home ownership, since you're so intent on proving my point for me?