Imagine you have no idea about aeronautics, controls, lift, thrust, where lift exists, how it works, etc., etc. You have no idea how to actually fly the thing.
Or land it.
So you build your best guess at what a working glider might be. You don't have robotics or radio to remotely control it, so someone's got to fly the thing.
You build a kite flinger and/or ramp or rail to toss it off the side of a building or cliff or whatever.
And then you hope, desperately, that you weren't too terribly mistaken about a great many things.
Franz Reichelt wasn't quite building airplanes, but he turned out to have misjudged his own competence in a somewhat similar manner:
Now consider the same situation, except that you've at least got a motor to buy your way out of design and/or piloting errors, at least a little bit.
Under which of those scenarios do you think actual useful design might have progressed faster?
Keep in mind that people have had the notion of flying with wing-like contraptions dating to the ancient Greeks (see the legend of Icarus). There were numerous inventors who threw themselves off hills or cliffs with various attempts. Lack of power, and the frequent short professional career track of such inventors tended to stymie progress.
Successful gliders and sailplanes almost entirely postdated powered heavier-than-air flight. The first designs appeared after WWI, and practical use didn't appear until the 1930s. My understanding is that popularity of recreational gliding didn't emerge until the 1950s or 1960s, again benefiting from aeronautical engineering, materials, radios, much better knowledge of aircraft operation, controls, and instrumentation.
Glide ratio is one measure of aerodynamic efficiency and sophistication. Early 1930s gliders achieved about a 1:17 ratio. Most modern gliders exceed 1:30, and the best 1:50 or more. This expresses altitude loss per unit foreward travel (e.g., 1 meter loss for 30 meters forward flight).
Google's Ngram viewer is a somewhat fickle guide, but suggests an initial spike in mentions in the late 1930s / 1940s, again in the 1950s, then a third in the 1970s:
There may have been earlier terminology used, and "glider" has other meanings which might confound matches, but at least for "sailplane", the trend line lags "aeroplane" and "airplane" considerably. (I've multiplied "sailplane" results 100x in this plot):
