Because a low TTL means that a reasonable % of page requests are going to see not only the latency of your DNS server, but also that of the shitty ISP's fetch s

Because a low TTL means that a reasonable % of page requests are going to see not only the latency of your DNS server, but also that of the shitty ISP's fetch store return cycle, which can result in a couple hundred extra ms on top of your page serving time. This extra bit of latency can make for a bad user experience, so they may not come back (every little bit counts). Because ISPs are likely to ignore it anyways and impose whatever _they_ think is a good value.