I don't know about logic but certainly people like to "celebrate" almost anything nowadays. Any excuse. Even if said celebration merely involves doing exactly what you would ordinarily be doing in any case. In this instance, supping a few quiet, meditative beers, having a jig, a punch-up and a decent curry, I suppose. Followed by a few more beers.
And, if you're English, these simple, unambiguous pleasures are a very big deal indeed.
Of course we can, and do, do the whole pageantry, pomp and ceremony-stuff at the drop of a hat any day of the week anyway, so it actually makes no difference to anything. Certainly, no special significance need be attached to the fact that it happens to be St. George's Day.