Agreed completely, but let me be devil's advocate here.
If he accepts responsibility then boeing is presumably fully and immediately legally liable. I guess that, particularly under US culture of suing anything that moves, everyone will try to cash in to the maximal extent possible. Including airlines possibly cancelling their existing orders - might they cancel 4,700 plane orders? What would that do to the company? By how much would boeing's liability increase on a straightforward "mea culpa"?
I can't agree with his butt-covering but I can see why it's happening. There seems to be little upside to accepting the blame. Could it destroy boeing? How many jobs lost?
"Boeing employs more than 153,000 people across the United States and in more than 65 countries" <https://www.boeing.com/company/general-info/> and a whole long-tail of US jobs indirectly.
How would it affect the US economy and standing in the world (I mean, more than it's already doing now)? I'm sure he's being leant on from above.
etc.
(please note I'm presenting a possible explanation, not endorsing it. There seems no moral way out of this. The real cause is the historical stupidity that allowed this to occur, for which he should definitely swing, and a few more suits. I am NOT endorsing these things I wrote but they seem things he will have considered. NB am a brit so I'm looking at this from the outside).