I’m not an engineer but I am an architect and I have worked closely with engineers on skyscrapers. Drilling all the way down to the bedrock is risky. When you drill a deep pile you need to use bentonite slurry to stop the pile collapsing in on itself, the pile auger is withdrawn and then a long pipe is used to introduce concrete from the bottom of the the pile which should displace the slurry out of the top because it is less dense than the concrete. However, this process doesn’t always work and you get bentonite inclusions in the concrete which are found when the pile is tested ultrasonically afterwards. I don’t remember the stats but lets say it’s something like 1 in 10 piles fail the test when you drill to 50m deep bedrock through clay. You then have to drill two more either side of the original pile and bridge over the defective pile but these each have the same risk of inclusions, so there’s an increased risk the workaround will fail. As you can imagine this can get pretty complicated and expensive quickly. As an architect I was involved because we needed to have a backup plan for how the basement would be reorganised if there were a lot of piling failures requiring extra bridging structures.