I think I finally found out why this is often the case - took awhile to figure it out, and many people may intuitively know this without being able to articulate why.

There is a lifecycle in industries and within companies. The more the industry you are in is exposed to competition, the more obvious and unavoidable it is. Some, such as a Dr's office, or a field/industry with geo-local niches (a plumber, or a well drilling outfit, or a realtor), the less obvious it is/the more you can sometimes avoid it. The more you're part of the global marketplace (tech, manufacturing - especially mass manufacturing, food, etc.) the more obvious and unavoidable it is. The way this typically works is:

1) A new field is discovered, a major disruptive shift happens in a marketplace, or a new niche opens up. Everyone jumps on the new openings - new players often have an advantage as they can move quicker and are less 'stuck' in old ways. They also tend to be small. They also tend to be pretty inefficient, but the opportunity is rich enough, it usually has plenty of room - most companies that aren't complete disasters will thrive. Even those that ARE complete disasters can survive.

2) Companies that start iterating on more efficient ways to produce value (more value for less cost) get a lot of extra capital in cashflow, or those that seem most promising are able to leverage outside capital (investment) to increase in scale. This means the 'machine' consuming customer needs is able to take more input, produce more output at a given company. This machinery is expensive to operate, but a key part - produces economies of scale. This rarely is at any one company, it is often spread across many companies, with various degrees of success/copying. With any luck, this expands the market (more customers, more money coming in). This often results in shifts of customers, as those who have free capital have invested in serving the customer needs better or are able to provide lower prices due to their improved economies of scale. This is why folks get so worked up about market share.It is also why Monopolies are so dangerous.

3) This iterates for awhile, until the market stops growing much. You now have a set of companies with various degrees of market share, ability to execute, economies of scale, debt load, etc. Usually 1-2 that are REALLY good, and others that give them a bit of a leg up in a particular niche.

4) Now it gets hard - market stops growing, or even worse shrinks. You have a set of companies with great economies of scale who can cut prices (easy for them, hard for others without that scale). These companies are probably also pretty decent at execution as they have a lot of practice. They also probably have a lot of capital (due to excess from their operations - 10% of a billion is a lot more money than 10% of 1 million, even assuming similar margins), or have experience raising capital. You also have a bunch of smaller companies with none of those things, or that tried to get there, couldn't pull it off, and are loaded with debt (even worse).

Guess who loses? It is rarely the big co.

Long term, it's called a cycle because of the shifts in markets/opportunities. They make the parts that were the big corp competitive advantage in the old environment - all the CapEx sunk cost + operation tuning at the big co - into more of a harm than a help in adjusting to the new reality. Also, no one wants to kill the 'goose that laid the golden egg', so there is intense organizational pressure to even attempt a pivot. Kodak and their huge miss re: digital photos is a great example of that.

This also comes out a lot in things like forest growth, crop development, etc. If you're not racing to be the tallest tree around, or the tallest ear of corn - you're going to get starved out by those who will. Unless you're lucky enough to get a niche anyway they won't try to go after.

Intentionally setting a 'be the best, don't try to grow too fast' business strategy is not, IMO, a good idea to do blindly, any more than settling into your current job and not seeking to learn or grow career wise or eventually find something better. If you're 'lucky' you may end up stagnant and complaining about all the people who passed you by.

At the same time, job hopping between FAANG's every month will quickly run out of steam and leave you with nothing you've really accomplished, and impossible to meet deliverables (like a company that has overpromised/oversold itself without figuring out things like execution, sustainable company culture, etc.). It's a balance.