There are two reasons for not "updating the spelling".
The first is that scholars are reluctant to tamper with an author's work; they do so only in order to make works which would otherwise be unintelligible accessible to a larger audience. There's no evident need to update "The Tyger" to make this poem accessible.
The second is that Blake was not only a poet but an accomplished visual artist—he made his living as an engraver—and he published many of his works, including "The Tyger" as visual compositions. These cannot be modernized if you want to encounter them as Blake presented them to his own public.
(This image of the page was published by the Guardian, credited to 'Photograph:British Museum'.)