Because you are the easiest person to lie to, yourself. Just because you work in a field doesn't mean your assumptions are valid. The more people (or you yourself) consider you an expert in something, the more you
should test your assumptions through studies etc. If you don't, then claiming you made a thing "for X" without any proof that X benefits from that means you've almost certainly lied to yourself, convinced yourself that lie was true, and are now perpetuating that lie by putting it on the internet. Especially when you do things based on your own experience, it's incredibly easy to forget that there are almost infinitely more people who are different and have different needs and experiences.
And of course, your things can be just fine, but without proof of that, claiming that they are just tells people you have no idea whether what you did was actually worth anything, and that undermines your effort.
The thing you made MIGHT actually be great! But it might also only be great for a super select few hyper-focussed people that you happened to ask for help (or not even, you might have purely relied on "your own past experiences"!), and be terrible in general.
Make tiny claims centered around tiny groups (e.g. "I/my customers needed something better so I made this"), not a problem. Make big claims involving "everyone" (e.g. "we made a typeface that improves legibility")? Back them up with proof. Show the studies where you've pitched it against a wide set of other typefaces, and have people of all visual impairment levels perform (also scientificially justified for this purpose) tasks that hinge on legibility.
(Because winning awards may be nice, but doesn't tell us anything. It just says "other people already liked this". It doesn't say anything about whether the thing you made is actually good)