Sophisticated attackers are too numerous and determined to get caught by perimeter defenses. It’s relatively easy to take advantage of vulnerabilities on the edge of the network or trick a user into granting access to their device. From there, attackers can lay in wait for days, weeks, or months until the time is right to spread to other critical systems. Organizations are better served by a security team that shifts its focus from preventing all attacks (which is pretty much impossible) to stopping the spread of attacks once they make that initial breach.
Source: https://www.helpnetsecurity.com/2021/07/12/2020-threat-landscape-see-what-evaded-perimeter-defenses/