You’re kind of ignoring a handful of things... sure, hohmann transfers are easy... efficient hohmann transfer are tedious, and can be difficult to align. Sure, I just sent a random craft out of kerbin’s soi, and was able to get flybys with both Moho and eelo... but they were inefficient transfers, and I didn’t have enough fuel to perform an orbital burn, or make it back to kerbin... I was just screwing around with craft design, in sandbox mode, so I didn’t really care about round trips... but you can’t just launch and “find a window,” because there are lots of “windows,” there are a handful of mathematically optimal windows... especially if you have a specific mission plan.
That said, I find it kind of silly to object to dV cheat sheet, and other online resources... for example, all science is built upon the discoveries of past scientists (We stand on the shoulders of the giants that came before us). Transfer window utilities are the most basic of which... the mathematics have existed for centuries, and you shouldn’t have to rely on trial and error, or your own math skill, to figure it out. When we start getting into atmospheric altitudes, the game is a bit thin on utilities, and there is no in game manner of determining aerobraking altitudes (it also seems to vary from craft to craft, and person to person)... so this is something that you’d either NEED to use an online resource or trial and error... if there are particular objectives you are planning, like a Duna landing, and return trip, you’ll never know how much dV you’ll need to reach Duna orbit, from the surface, without the resources (unless you wanna plug through all the math)... so, it is pretty much up to you... I feel like these resources are perfectly fine, because the game is more about craft design and piloting, and not your ability to generate pork chop plots. I’d say, it’s cheating once you start using other people’s designs... however, nothing wrong with other people’s designs inspiring your own...