This (purported) proof is 500+ pages. For a mathematical proof, that’s enormous. Sampling a few of the issues of the journal in question, its longest papers seems to be about 40 pages (
http://www.kurims.kyoto-u.ac.jp/~prims/pdf/43-1/contents-43-...,
http://www.kurims.kyoto-u.ac.jp/~prims/pdf/42-3/contents-42-...)
And those are papers that an expert can skim to get an idea about the flow of the proofs. This one, if you jump in, says you things like “every fooable bazz is a bar”, where foo, bar and baz are new terms no mathematician has any intuition for that, likely, got defined in terms of other hitherto unknown terms qux, quux and quuz, making it impossible to judge whether that statement has merit, or how it leads to proving the abc conjecture.
And yes, an automated proof checker would be nice to have, but we aren’t there yet, by a wide margin. Getting there would make this proof a lot longer.
(For a loose analogue: imagine that the published solution to a “mate in 2” problem in chess wouldn’t just be “Queen g1, check, King h8, rook h2, mate”, but “whites queen is on field d1, by rule r queen can move horizontally across empty fields, there are no empty fields between d1 and g1, that doesn’t bring white’s king in check (blacks pawn on e4 can’t take it because…, blacks knight on c6 can only move to… because of rules…, so white can move their queen to g1. That brings black in check because of rules…, etc)