Hand scraping is only as good as the surface you are using as the master.
In machine grinding the wear on the wheel has to be taken into account as well as the trueness of the slides on the bed which will change with use.
In hand scrapping the master surface has only minimal wear if any.
Hand scraped surface plates are made in 3's in which each in turn is used as the master so finally you end up with 3 plates true to each other.
I assume by now you could get a machine made hardened plate that is better than a scraped one but I expect that you would pay handsomely for it.
Just my 2p worth
I have tools I don't know how to use!!