Tapering a surface is a really good idea aerodynamically. Having a uniform TE is also a very good idea aerodynamically.

People got into leaving the TE square for a couple of reasons.

1. It's a lot easier to create a uniform squared off TE than any other shape.... and a uniform TE is good.
2. A squared TE is a lot easier to maintain. It's strength compared to a sharp TE is considerable and normal hangar rash is nothing to a squared TE but ruins a sharp one.
3. A lot of people are dead lazy and have little or no building skills. OK.... that's harsh but it's true, isn't it.
4. Those same people have little or no aerodynamic knowledge other than what they got from the loudest experts at their flying field.
5. The loud experts are described in #3 and #4 above. (Like I said above, this is harsh, but it's true also, isn't it.)

So the way square TEs came about:
Long ago when only kits or plans were available to the hobby, some of the newbies who hadn't any knowledge of aerodynamics didn't know to taper all the surfaces. And when they showed up at the field with these squared off things, the lazy guys noticed them and after seeing them fly, realized that they didn't have to go to all that extra work. Now, when those same lazy guys showed up with THEIR squared off ugly things and were questioned as to why they'd done such aerodynamic foolishness, they stuttered and spluttered and then bs'd something about Reynolds numbers and how the decalage of the empennage and the wing volume all contributed to an imbalance that could only be cured by moving the abscission point aft of the TE and that's why they had square TEs. And all the lousy builders who hadn't thought of cutting corners (pun intended) by sluffing past the effort of tapering, saw that they could actually come closer to making square TEs look halfway good and they started doing it too. And it caught on.