Speaking as someone who (for a time) worked in the IT department of British Airways, the culprit is really just ancient systems that aren't well maintained, along with institutional and industrial pressure not to improve them or upgrade them.

For example, I worked as part of the team that managed the software that allowed pilots to submit flight plans. Any upgrades had to go through multiple weeks of reviews and testing (I don't mean code review - I mean reviews through managers and processes), and was run on some rather ancient hardware. Moreover, thanks to pressure from the pilots union, the system had to be able to accept flight plans by fax, so had a lot of legacy cruft to support that too.

The problem isn't agility, corner cutting or moving too fast - it's moving too slow.