Two factors come into play here:
practicality and
motivation.
Designing and building a fighter, even a low-cost one, is an extremely expensive and technologically challenging endeavor. Mission creep, overambitious specifications and poor management can dog programs from the very start. Plus, there are a host of technological obstacles to be confronted. Complicated programs with complicated products rarely travel simple (or cheap) paths. Add to this the potential for political bickering and interference, and you're looking at potent mix that will confront anyone trying to develop and market an "F-XX" light fighter of any kind.
Now, does this mean it is impossible to do? No. Many nations have built or contributed to modern fighter aircraft in the past and many will continue to do so;
but the challenges at hand can only be approached and overcome if there is motivation to do so.
There has to be a clear and present market for the F-XX. Without it, most manufacturers will shy away from the risks involved.
I'm not convinced a sufficient market exists at this point in time; nor will it 10-20 years down the road, when the products of a fighter development effort started today would start rolling off the line. To be fair, there certainly are buyers out there, as events like Iraq's recent efforts to
purchase 36 F-16s shows. However, with the F-35 and various 4.5-generation strike fighters snagging contracts in India, Brazil, and Europe, much of the market an F-XX might compete in is already flooded with vastly more capable platforms. And even in the markets an F-XX might perform well in (moderately wealthy countries without the need or the means for F-35s, Gripens and Rafales), there's still plenty of competition. The slew of retiring USAF, ANG and international aircraft provides potential customers with hundreds of F-16s, various Mirages, and even a few Viggens. These secondhand fighters offer a range of advantages over a brand-new F-XX.
Firstly, many of these aircraft have already been integrated with Western weapons, electronics, etc. This allows buyers to minimize new equipment buys, by allowing them to use ordnance they have in stock. Second, inexpensive spare parts are widely available; something brand-new fighter designs would struggle to compete with. Thirdly, F-16s and Mirages are proven aircraft, giving prospective buyers a concrete understanding of what they would be getting. Not so with the F-XX. It’s hard to beat 60+ combat kills with an airshow brochure.
And while these used aircraft may be aging, aggressive SLEPs could add years to their effective service lives.
I third issue I see here is the fact that not all demands are created equal. Different buyers, although they may all want a "simple, low-cost fighter" often need (and want) a variety of vastly different capabilities. India, for example, demands its combat aircraft to pass stringent high-altitude operational tests (see the Leh tests undergone by the MMRCA competitors), something a sea-level Middle Eastern country would have little need for.
Fighters (especially small ones) can only be so flexible, and I strongly doubt a single tiny fighter can do everything dozens of diverse air forces would demand of it.
Apologies for the wall of text. There's a lot to be said on this subject.