What backfills the motivation to do this is:
1) who is behind it, actually.
This isn't about dogman144 making CTO. It's BlackRock/private fund/whatever steering companies of interested towards long term, favorable outcomes, at the cost of the salaries for 20 people mole-strike team, tough NDAs, a large finder's fee at the end, and at the benefit of 0% of the negative publicity they get for doing this sort of action out in the open. No % shares reporting anymore, no PR, no protests for buying up residential real estate in bulk, and so on.
2) how much does the figurehead mole make, as this could be an excruciatingly stressful experience
Start with a fee model like 2/20 for hedge funds, and data like FireEye is getting bought out in "an all-cash transaction for $1.2 billion." At a 3% mole fee of the final transaction, thats a $36mil (edit, math :( ) payout for the mole, using FireEye numbers, putting aside 5-7 years of double-dipping Raider pay, FireEye pay. Kraft bought Cadbury for $19B. Some of the larger LBOs push $45B. There are a number of ways the incentives can work here - if this takes less time to make $3.6m than an IPO would, more surety of outcomes because you have a titan of a fund steering things, and guaranteed Partner at the fund once this is complete, and so on, the numbers likely make a bit of sense.
3) Jumping companies: also doable. Either way the mole team wants to play the long game: they really want X-company, and to do that they could do a few years of jumps. Few years at a single company, or a few years at separate companies until that "senior" mole team lands at the target company - same/same.
You nail it with this: it's a great way to get assets inside the upper echelons of a lot of major companies. And add in the above, the payouts look good too.
Is this legal, do you get on the wrong side of outside employment regs at the company, and so on? Not sure! But what's interesting to me is with remote work, this suddenly gets a whole lot more doable, an there are a lot of all-remote, pretty valuable companies (revenue, or IP access - see GitLab) out there. This puts aside all considerations of intel agencies doing this, as well.