Technically they did get funnier over the years. The Simpsons is like SNL, they many different eras of writers, and if you go watch the first two seasons, they are kind of unfunny. Homer used to say Frosty Chocolate Milkshakes every episodes
But the show made a major shift in season 3. First off, the writers realized that cartoons are capable of taking satire in a whole different level. Secondly, FOX decided to switch up staff, because the show was growing in popularity, FOX gave them better writers and producers.
Now here are Season 4 Simpsons
Even if you don't like that style of humor, you can't disagree, The Simpson during this season 4 - 11 period, had the most poignant writing. Primarily because The Simpsons, at that time, could tell a genuinely good story, and still make it funny. Watch "Who Shot Mr.Burns Part One" (it doesn't matter if you know the ending), if you see it again, you will find a concise story of community trying their best to combat a evil, and a man's descent into supervillain madness.
I can't say modern Simpsons is better than Season 4 - 11 Simpsons. It is better than season 1 and 2, but The great writers have moved on, most of the good ones went to Futurama, hence why both had such good subtle humor. The modern Simpsons doesn't seem so concerned in writing a great stories, rather than just making you laugh. Its still funny to me, but its like a Will Ferrel movie, funny but not engaging. Whereas Seasons 4-11, was like a Woody Allen movie, funny and subtle, but still capable writing a story that could make you think.
Now to end with one of my favorite Simpsons clips