It sounds like you want OIS simply because of marketing. First of all, not all pro cameras have OIS; lenses typically come with or without image stabilization depends on if you are willing to shell out more money for ones with it. And there are times you WANT to turn it off because it does interfere. In photography,
typically if your shutter speed is higher than 1/focal_length (second) of the lens you don't need image stabilization and even having it on is not beneficial. Pixel's lens is about ~27mm in 35mm term, which means the software is probably tuned to shoot faster than 1/28s internally. In addition, for hdr to be effective, every consecutive shot needs to be done as quickly as possible to avoid ghosting. Another point, due to this sensor supposedly having low ISO variance, the amount of noise you get by under expose (keeping shutter speed high) and then fix the exposure afterward or slowing the shutter speed for the proper exposure and don't fit it afterward is the same result in terms of noise.
TL
R Basically they have done everything there is to do to make inclusion of OIS useless in auto hdr mode. And as a side effect of keeping shutter speed higher, you have better chance of capturing moving subject in low light without the motion blur which arguably is a better choice for average Joe's usage pattern.
OIS will help if you get out of auto mode and want to take long exposure yourself. But if it's on a tripod, good thing it doesn't have OIS because that is one of situations where you want to turn off OIS and no phone camera I know of offers that option.
How many people using Pixel are going to be shooting in manual, long exposure mode that will make OIS useful?
edit: That's not to say they couldn't have included OIS and tune the software work with it in tandem BUT you do lose some benefits of the current approach. It's not a complete win.