Why does this toilet clog so often?
Troubleshooting steps/questions:


Has this toilet always had this problem, or was there ever anything installed in this location that didn't clog like this? If the toilet has always had this problem then it's likely some issue with the initial installation; the drain pipe diameter may be too small, or there may be insufficient venting which is causing slow drainage in the wall because there's no "air behind water".
If it didn't always have this problem, then something has changed. Would that be anything obvious like a remodel, or something more gradual like build-up accumulation? If the toilet was replaced at some point, either as its own job or as part of a larger remodel, then most likely something was done during construction/installation that is causing an obstruction. If not, then mineral or organic build-up is the most likely culprit.
From the comments, what DID the plumber do, if he didn't scope the drain, lift the throne, and/or inspect the vent stack from the roof? Anything he did do, besides flush a toilet with nothing but clean water in it, may help to eliminate or diagnose potential problems.
Does the toilet always seem to drain slowly, even if it doesn't clog outright? Slow drainage of the bowl in all circumstances would point to an obstruction of the pipe diameter itself, such as an improper flange installation, a defect in the toilet, or blockage in the piping. Fast flow in a "normal" flush, but easy clogging, generally points to a foreign object blocking the pipe in such a way as to trap solids as they go down the drain, but allowing water without any solids to pass by normally.
Are there small children in the house? A child could have flushed a pencil, pen or other long, stiff object which is now caught in a turn in the plumbing and is trapping solid matter to form clogs. Scoping the drain will identify this blockage, and a professional snake will clear it (the homeowners'-variety snake with a coiled end won't do anything; you'll need a snake with bladed ends designed to chew into or through solid blockages, which require some knowhow to use without further damaging your drains).