there are still a very high percentage of players who insist that sealing a blade is changing its qualities. Where would these players go to purchase their blades ????
In addition to which, sealing a blade can come with it's own problems if not done carefully or properly.
All blades have wood in them, which means by extension that all blades 'move' (ie: wood in the blade is constantly exchanging moisture with its surroundings due to changes in humidity --. the wood swells up when absorbing moisture, and shrinks when releasing it. Movement can occasionally cause wood to bow or warp if the expansion isn't equal and even throughout the wood, or if the blade is made of different wood species that have different expansion rates.
It's impossible to stop wood moving all together, unless you encase it completely in a thick layer of impervious plastic/epoxy/PU etc. Many wood finishes therefore aim to preserve the wood (and slow down movement) by minimizing water absorbsion, but not by trying to eliminate it completely (because that can also lead to problems).
As sealing a blade radically slows water movement between wood and air, you need to ensure that it is sealed with the 'right' amount of moisture in it to begin with.
Once sealant is applied to the whole blade, it's much harder for that wood to equalise. You therefore want it used in a playing environment with a narrower range of relative humidities (one that is closer to the residual moisture level of the wood).
The best way to ensure THAT, is to have the blade sealed by the user, once it has reached its final destination (and hopefully once it is fully acclimated to local humidity levels).
Seal the blade at the factory during manufacture however, and you risk the blade potentially cupping or warping if it then ends up being sold in a place with lower/higher relative humidity.
If you read any carpentry blog / magazine article about minimising or managing movement, you will see them recommend that timber be rested / fully acclimatised at its final destination, for at least a month before sealing it. Table tennis blades are typically not sealed at the factory for much the same reason.
Nowadays with my own custom blades, I only seal them "at the factory" when I'm sure it's going to a location with a similar humidity range to my workshop. Otherwise I either store the blade at home at a different humidity for a while, and then seal it (and even then, I only ever seal the playing surfaces and handle scales). Otherwise I recommend the owner seal it themselves once acclimatized to its new destination.