Sometimes they do, very successfully. Several prominent tech companies backed efforts to neuter patent trolls in the prior decade, to positive effect. Patent trolls are one instance where big tech's interests are largely aligned with smaller tech companies, as both hate them.
In the recent case of TC Heartland vs Kraft Foods, some large companies (Apple, eBay, IBM, Microsoft, Intel, Walmart) supported the effort to abolish being able to file patent lawsuits in most any venue (a concept which helped lead to East Texas becoming the capital of patent lawsuits in the US). Often you'll have big companies fight eachother in situations like that, as big pharma was on the other side of that, arguing in favor of being able to sue in any venue.
Why would they want to do that? They have the money to play the game... it's startups and disrupters to their business who don't have the money to do it. When the day comes when a startup challenges one of these companies, one of two things will happen: they'll be forced to sell or they'll get sued to oblivion. Just the threat alone keeps startups from operating in their space and getting investors, etc. These companies don't want to compete with you.
I think because a) this is a huge additional investment b) they still have to invest in patents until (if) the system changes c) then the whole playing field is changed, and that's also a huge risk
Status quo is almost always safer if you're one of the biggest.
