The usual answer is that activists have a short-term focus. But while that is sometimes true, it's not the real issue. Jeffrey Gordon makes this point succinctly in a recent post:

When we examine the behavior of institutional investors who are the majoritarian stockholders of the largest public firms, we learn that the same investors who purportedly follow the activists’ siren song for the “short term” also turn over large sums to venture capital firms and private equity for investment in promising companies over a ten year commitment period. This is the very definition of long term investing.

Instead, the real issue is whether activists are able to come up with strategic plans for portfolio companies that are superior to whose of the incumbent board and managers. Gordon thinks they are:

Ownership of large public companies is now re-concentrated in institutional investors – pension funds, mutual funds, insurance companies — which have the capacity to evaluate competing strategic alternatives for portfolio companies. Now turn to an activist’s campaign, which starts with a claim that the current management is making serious operational or strategic mistakes, reflected in the company’s underperformance. Institutional investors have the voting power to determine the outcome; how should they respond? To start, institutions increasingly have come to understand the activist is sincere in its belief about problems at the “target,” since it has made a significant upfront investment and has a business model that depends upon repeated successful engagements. ... In short, the present wave of shareholder activism shows us that the current corporate governance infrastructure is creaky, a swaying bridge that needs renewal. To cast this as a debate over “short term” vs. “long term” misunderstands a genuine problem.

This is where I (respectfully) disagree. As I argued in my essay Preserving Director Primacy by Managing Shareholder Interventions (August 27, 2013), available at SSRN: http://ssrn.com/abstract=2298415: