I've played 250 games on Teemo with 8/10 games building AP shroom over on hit: https://na.op.gg/summoners/na/taehe%20is%20faraz

Here's my take:

1. The higher you move up the ranked ladder, the more punishing people are with your mistakes, games get closed out faster. Sure, there will occasionally be some that creep out to late game where shroom value gets much higher but more often than not, people capitalize on early leads and you don't get a chance to "scale".
2. Follow up to #1, on-hit build lets you snowball hard very early. If you rush Nashor as your first item vs say, Liandry, you're going to be funneling most of your resources into dueling/split pushing process rather than teamfighting.
3. Newer champions (with dashes, escapes) plus the introduction of other items (stridebreaker, galeforce, etc) has really put a stunt on Teemo's laning ability imo. It doesn't make the game unplayable, but it significantly reduces the edge for Teemo so it's not forgiving at all if you go 3/0 in lane and then give a shutdown + get Herald'd.
4. People have gotten better with vision control over the years. Maybe this is anecdotal but even just a few seasons ago, I would throw shrooms into minions, people would not expect the AOE, just stand to it and get hit. Nowadays, almost everyone walks away from the AOE impact and everyone just buys sweepers. Most of my games, enemy team has 4 or 5 sweepers and unless you distribute your shrooms very efficiently (easier said than done), they effectively are negated

Bottom line - can you make AP Teemo work? Absolutely. It's all subjective and I really think it comes down to playstyle preference, but with the current meta, PTA is probably the most "optimal" build at the moment. I put that in quotes because it doesn't necessarily mean all the time. For instance, you're not going to be able to proc your PTA or get a lot of free hits on a Malphite during laning or say, Ekko jg.