Now please explain to me why a CRZ thrust limit(CRZ) is set as default rather than MCT?
That's a good question.
maximum cruise thrust
This thrust rating is used as a reference for the thrust available for cruise. It is not a true limit on cruise thrust, as the engines can be advanced to maximum continuous thrust in an emergency situation. It is usually designated as MCRT. It’s usually a slightly lower thrust rating than maximum climb thrust, although in some instances the two are the same. There is no time limit on the use of maximum cruise thrust.
Cruise is normally conducted at a thrust setting somewhat less than maximum cruise thrust. MCRT is the upper cruise thrust threshold for normal operation. Should the pilot wish to cruise at the fastest speed possible, he may advance the thrust levers to the MCRT setting and accept the resulting speed provided, of course, that it doesn’t exceed the maximum certified speed VMO/MMO. These are discussed in the chapter entitled “Speeds”.
Because maximum cruise thrust is not the basis for any performance level mandated by the aviation regulations, the data for MCRT doesn’t appear in the Flight Manual, but rather in the PEM and the QRH.
Just a guess: it's just about economics. Most of the time, you don't need MCT, so if Boeing says 94.6% is the limit, the engine/airframe will be that much more "fuel efficient." That 0.00001% of cruise at MCT costs more fuel. When marketing is digging for every dime, they can sell a less expensive CASM than the
other guys.