Because there aren't. That's the one piece of gear that's missing from the modern baby world. We've got modern strollers (Orbit, Bugaboo, Phil & Teds, Quinny), modern car seats (Clek, Orbit, Maxi Cosi), modern highchairs (Nest, Oxo Tot, Bloom), modern bouncers (Bloom, Maclaren, BabyBjorn), modern playards(Phil & Ted's, BabyBjorn, Graco's element) and even a modern swing now with the Mamaroo. Bloom, Nursery Works, DucDuc, Monte and a ton of others have the furniture market covered.
Skip Hop, Oxo Tot, Boon and Beaba make modern dishes, spoons and sippy cups. Hoppop,4moms and Boon brought us the modern bath. There are so many modern baby toy companies that I can't possibly name them all, and carriers and diaper bags are also aplenty.
Yet a simple search for a modern baby walker, excersaucer, entertainer or jumper will get you nothing more than this prototype from Wonjune Song (Thanks BuyModernBaby). Why hasn't anyone created a modern entertainer yet? I think there would be a huge marker for it, seeing as no others exist. I guess we will have to keep waiting though, because I haven't even heard of a rumor of one.