People say that modern airliners are more resilient to turbulence, but I see that a 707 and a 787 still have the same G-rating. Why is this? The 707 was introduced in the late 50's, the 787 in 2009.
Back when Air France 447 had recently gone missing, many were speculating that the aircraft had broken up in the turbulence associated with the massive thunderstorm it was flying through. Some aviation spokesperson, who I believe were associated with Airbus, said that the planes are designed to withstand the worst storms with a 50% safety margin.
Several airliner models from the 50's and 60's have broke apart in flight due to turbulence associated with thunderstorms (A 720 B, A BAC-1-11, and a F-27 in 1981). There were also breakups involving clear-air turbulence (707, Mt. Fuji, 1966, and a Consolidated Airlines F-27). I checked to see if their G rating was lower than the planes of today and they were the same - 2.5+ G Limit Load, with a 50% safety margin, so 3.75+ G Ultimate Load (beyond which structural failure occurs).
Why is this? Shouldn't the G rating be much higher if modern airliners are more resistant to violent turbulence?
Note that the 2.5 g limit is for load due to manoeuvring, not only for turbulence. The certification specification for large aircraft on the subject of turbulence and gusts has changed several times since the certification of the Boeing 707.
Turbulence, and its effects on aircraft, is nowadays much better understood. Different aircraft react differently to the same turbulence. What will result in a high g-load in one aircraft, may be a moderate g-load in another. Therefore the certification regulations now focus on prescribing the type of turbulence that needs to be safely dealt with, instead of the resulting load factor.
In 1964, a formula was introduced describing the gust load that the structure aircraft has to deal with. This was added as FAR 25.341. This section has subsequently been updated in 1990, 1996 and 2015.

Modified from my earlier answer to a related question