Years ago I joked to a friend that the bull market would only end once John Hussman closes his fund... and to date he's still hanging on.
In all seriousness, these are the factors you have in play:
1. Everyone and his brother is just waiting for this historic bull run to end so they can buy on the cheap, and justify a decade of under-performance. Everyone is looking in the rear view mirror at historical cycle durations, yield curve inversion, etc. etc. and thus the almost universal expectation is that a recession (+bear market) will incept in the next 2 years at most. Is there anyone in the biz who thinks the U.S. market can grind out 5%+/yr through, say, Jan 2026?
2. CBs are keeping risk-free rates in the basement with no sign of an inflationary breakout. Yet at least until very recently, the universal belief was that we were much closer to the "bottom" than to the "top" of the interest rate cycle with meaningfully higher rates to come in the near future. What happens if we are actually at the top of the rate cycle, and interest rates never go higher than they are today through, say, 2050?
3. There's lots of talk thrown around about the "everything bubble", prices distorted by free money, imbalances building up for a thundering crash and so forth, but nobody can point to a specific clear speculative bubble with systemic risk implications.
4. Many automatic and structural processes keep up a steady flow of price-insensitive money into world asset prices, including: stock buybacks, 401(k)s and other forms of automatic passive "deduct from your paycheck" investment, migration from active managers to robo-advisers and passive vehicles, widespread adoption of sovereign-wealth funds by resource-producing countries, and the constantly increasing share of income and wealth controlled by the top 1% who, year after year, sock away their entire surplus in pro-managed family offices.
The result of all this is what we've seen for years, which is that growth keeps chugging along in a narrow slow range (and it's going to slow down even further now that we have full employment) while equity prices keep on marching higher, punctuated by occasional washout moves to flush weak-hand trend followers, poorly-thought-out quant strategies, and other pockets of crowded positioning. Then the automatic flows do their thing and the market is right back up at ATHs a few weeks or months later.
To really change this situation you need one or more of: serious political turmoil (like WW3, or a shooting civil war in the USA), a breakout of high inflation forcing the Fed to hike rates dramatically, or an unstable speculative bubble on the order of 2000 or 2008. The 1982-2000 secular bull ran for 18 years before the tech bubble and crash broke its momentum, this one can easily go longer.