Those who attain power are corrupted enough to seek to hold on to it; and spying on those you govern is one of the ways to hold on to that power. “What are they up to? Scheming? Against me?”
It's ironic because if politicians do a good job, and don't have much to hide, we'll trust them more and re-elect them.
You can't have wealth without power, because then I can come over and take your wealth from you. Power through violence (monopolized by the state), is what guarantees your rights to your wealth.
The wider the wealth inequality increases, the wider the gap between the haves and the have-nots becomes, the more threatened the wealthy feel and so the more draconical surveillance and speech law enforcement crackdowns becomes on the masses. See modern day UK, EU, etc.
In my EU country the government increased the budget and drill training frequency for riot police this year even if they've been no major events to necessitate that. They know what's coming, since more and more people are poorer from government actions, and they're preparing already to crack down on what's to come. I assume Pegasus and other such spyware to spy on us to prevent descent, is part of that budget too.
So power beats wealth. Easy to see these days with today’s wealthiest people on the planet cowering and paying tributes to Trump.
Also, power is convertible into wealth, as countless politicians have proven, while wealth is not necessarily convertible into power (like Bloomberg’s failed presidential bid has shown).
In conclusion - it’s about power, specifically political power, and the individuals yielding it: the politicians.
Power is just the ability to make more choices, and make bigger choices. Power is, really, the exercise of decision.
The more money you have, the more choices you can make. You can choose to buy a yacht. I can't, the decision is made for me - no.