+many. There are plenty good Enterprise software with bad rep. You can spend a lot of engineering talent to rebuild what is already solved or just buy boring tech. Splunk by definition is boring and with Twitter scale makes sense. Twitter is not too big, its decently sized.
Also, when you have bespoke log solutions and engineers leave, you end up with unsupported solutions. This may be Twitters 4th logging solution IIRC. Plenty of people know splunk. Loglens, not so much.