Activist Investing

Possible reasons:

1. Just because an activist fund initiates a position does not mean the stock price will move meaningfully / immediately (some funds have more credibility than others / higher hit rate on activist campaigns, value creation story could be really clear or more of a "show me story", etc.)

2. Even if the stock does move (say +10%) the next day, taking your gains immediately would be less tax efficient than holding for >1 year (depends on jurisdiction)

3. Assuming #2 holds true, getting out of your full position could move the market depending on size of stake / liquidity in the name and reverse a lot / most of the "gain"

4. Perhaps the most important reason: activist shops typically run very concentrated books and will focus on very few opportunities where they think A LOT of value can be unlocked over time with their approach... say they think there's 50% upside in the shares from divestitures, operational improvements, management changes, etc.  Their "letter to the board" and disclosure of a stake may move the shares up 10% due to the market's expectation that some of this will play out, but there is another 40% predicated on actually "creating the value".  Activist funds will want to continue applying pressure on management, the board, and maybe bring their own resources to bear to create that value and capture the remaining upside (this might be a better use of time / capital than locking in the initial uplift, assuming that's even possible)

Of course if for whatever reason the market moves a lot shortly after the launch of a campaign and the go-forward return on capital / time is low (relative to other names in their portfolio / other ideas) then they may get out sooner but in general it's not as easy as saying "let's launch a campaign and lock in our 10% gain the following week" and repeat every week.  You can see how the credibility of the activist would decline pretty meaningfully, rendering the strategy ineffective.