For the sellers who, as an afterthought, have considered doing this, it would mean altering their listings - that could be a lot of hard work There is nothing wrong with hard work, but when it is unnecessary that is something else.
For the buyer who would like to make an offer, but sees no official 'Make Offer' button, they might think that to make an offer via ASQ would be seen as auction interference, and might shy away from this for fear of a 'slap' or account suspension.
While the about me page (and even including a 'seller notice' message) are other ways of making a 'make offer' available, I feel very few buyers check out the About Me page. The 'seller notice' message just doesn't stand out anyway.
For me, the way forward for this is to give the seller the option of adding Make Offer to their listings on a per listing basis, with a nice noticeable button. Those that are against the idea needn't use it.
For what its worth I made a few good sales on the dark side by making Best Offer available.
I've had buyers ASQ/message offers over here. I sometimes have to take a little less than I would have liked, but I prefer an empty space on a book shelf and a Sunday roast, rather than a book staring at me while I munch beans on toast after Sunday worship.