This has been puzzling me for quite some time - I really can't think of any good reason why many startups use third-party landing page services.
My thinking is that any startup that has got to a state where they're ready to collect/offer beta or pre-launch registrations/invites ought to have all the resources to offer this on their own site.
So, why use third-party services?
1) Someone with no coding skills can put up a nice looking page without needing a designer or programmer. Market testing an idea then becomes a lot cheaper. This happens before there is an actual "startup" with a dev team.
2) It could be cheaper/easier than building it yourself. From a managerial perspective, this could make a lot of sense. Option A is taking your dev team off the core product so they can build a landing page, integrate with MailChimp etc. Option B is paying a few dollars per month and having it done in an hour or two.