I'd summarize it by saying that the reason behind technology generally lacking in large companies is a result of mismatched incentives.
In large, established companies, a lot of processes that could nowadays be replaced by software and automation are done by people. Some of those people are senior employees and/or have a manager whose standing within the company depends on the number of people they manage. Some of those people may even have "money under the table" relationships with vendors of subpar solutions that will be jeopardized by a new in-house solution being developed.
The feedback loop in large companies is long enough that individual employees can make decisions that are beneficial to their individual situation at the expense of the firm's profitability, but that fact will only come to light in so long that by that time they will have moved on or retired.
Therefore the reasons behind tech being shit at large companies is typically a blend of:
* nobody within the company having enough expertise to truly vet the people/consultancies they're hiring, resulting in lackluster results
* people within the company know (or eventually realize) that the people they've hired are competent enough to put them out of a job, so they give them intentionally-sabotaged instructions to deliver something subpar as to protect their job
* the working conditions, pay and/or working conditions within the company are so bad that nobody competent enough to make a change walks through the door, so the only people that do walk through that door wouldn't be able to make an improvement even if there were no other roadblocks in the way.
In general, in my opinion, it is a combination of all these factors. Incompetence of the incoming talent is a big one (that benefits the people relying on the status-quo), but should there be a breakthrough employee that can actually make a difference, he will be stopped because of the other reasons (otherwise he'd put many people in a tricky situation and they don't want that).