The Soviets landed numerous probes in Venus that not only survived the landing but also transmitted data and even took photos for over 2 hours in the 60s and 70s. It's challenging, but it's definitely feasible. With the advancement of technology (we already have the technology needed for heat and pressure resistant electronics and materials), there have been quite a few concepts for a Venus lander (including both "simple" probes as well as rovers) that have been proposed; at least two of them were even proposed as potential targets for the next upcoming New Frontiers mission, but were unfortunately not chosen... That's why I'm a bit frustrated with another orbiter (for the mission in over 10 years, nonetheless!) while we do have the means but are seemingly not willing to push ourselves further.
What do you mean by saying that we can't go to Challenger Deep? We've been there before twice and are about to go there again with the Five Deeps Expedition! It's baffling considering how little media coverage that mission gets. The team already managed to go down to two of the five deepest points (in Atlantic and Southern oceans), are currently preparing to do the next one in the Indian ocean, the descent to the Challenger Deep is planned for May 19th and they will then finish the mission by going down to the deepest spot in the Arctic ocean in September. Can't wait until all that is made into a documentary.
So in both cases (Venus landing and deepest ocean trenches), I'd say it’s not the technology which hampers the scientists, it’s the funding.