More like "liquid ass", please forgive the pun. The core issue is that Apple et al all have their UX/UI in-house, which means on the payroll, which means they

More like "liquid ass", please forgive the pun. The core issue is that Apple et al all have their UX/UI in-house, which means on the payroll, which means they have to produce something new to justify their continued employment. But instead of devs who always have some bugs to fix or some old cruft to refactor, these opportunities are rare for UX/UI, mostly because you'd also need to employ proper support teams that have a way of aggregating customer complaints or improvement ideas (and legal FUD surrounding the question of "copyright on suggestions" - LWT famously bans their employees from even reading r/LastWeekTonight to avoid getting held liable if they use an idea someone suggested there) and to derive improvement ideas from there. It's either "boom (every 3-4 years) or bust".