The “30” year average climate is from meteorologists (in 1935) who selected “30” an average when reporting local weather, and comparing the current weather to an average that an adult could relate to. This was selected and defined as (climate normals) before modern computers and communications. Maintaining averages was a time consuming process -- using 30 years was a labor saving adaptation, standardized across countries. Using 30 years as the definition automatically creates trends over 30 year periods.
Demonstration of the difference between 35 year and 70 year linear regression using British Hadcrut4 surface temperature estimates at the link below: The green line (1910 to 1945), is pre-man made global warming. The slope is almost the same as the global warming slope from 1980 to 2015 (blue line, allegedly all man made).
The IPCC's claimed CO2 growth rate for the second period is over 3x larger than the first period CO2 growth rate, yet the warming trend is almost the same. Note that CO2 growth before 1958 is derived from air bubbles in Antarctica ice cores -- not very accurate, compared to real time atmospheric measurements in Hawaii since 1958.
In any time series of natural data, short time periods could give the false impression of a steep warming or cooling trend ... when they are, in fact, just part of a long, mild periodic cycle (100 to 500 years of intermittent global warming, or intermittent global cooling). Our planet has had intermittent global warming since the 1690s. Global warming did not start in 1975.
Both the Pacific Ocean (PDO) and the Atlantic Ocean (AMO) have 60 year temperature cycles (30 warmer years and 30 cooler years). The PDO is now in cool phase, and the AMO will soon enter it's cool phase. The change of ocean temperatures is very gradual.
It’s not hard to see why a 30 year "climate" cycle is preferred – it’s far more useful for generating a Climate Alarmist narrative.