Judging a tailor based on what they are wearing is a fundamental mistake and one only an idiot would make.
A tailor can't measure himself and certainly can't make objective adjustments to a garment he makes, so he must either trust an underling or another tailor to do that for him. In big houses that isn't a problem but for small one-man-bands it simply isn't worth the hassle. Not to mention that even without labour costs the cloth would run a few hundred pounds at trade price and not everyone (especially craftsmen working alone) will want to spend that money on their work clothes.
With regard to sizes, have you even measured someone for a suit? In a tightly fitted jacket you simply cannot move freely enough, let alone kneel down and fix hems etc. even if you could measure, what about a few hours bent over a pattern making adjustments? Tailoring is a basically physical skill, and as such you need the equivalent of overalls.
Beyond the practical reasons above, there is the simple adage that tailors generally dress poorly anyway. Alan Flusser (not a tailor, but regarded as king of menswear) is notorious for poorly fitting clothes, but the subjects of his advice always look good.
Not trusting a tailor because of his clothes is the same as not trusting a mechanic because he doesn't drive a Porsche. Just because it doesn't fit with your mental image, doesn't indicate that he is unskilled.