There ae two reasons. First, the models don’t match what has actually happened, and second, the Summary for Policymakers (SPM) misrepresents what the science says as reported in the IPCC’s AR5.

The theoretical models used to predict future warming are not consistent with atmospheric observations; the present models cannot handle clouds and other important climate factors properly. While the heavily-manipulated surface-based temperature record seems to approach the degree of warming predicted by these models, it is contaminated by local urban effects and covers only a small fraction of the globe. Satellite-based temperature readings are accurate and truly global, and they show a minor warming trend well below that predicted by the models.

The gap between computer models and observations remains an unsolved puzzle. The simplest description is that climate sensitivity to CO2 is close to zero, as demonstrated empirically. Why? Regardless of any unsettled science details, it seems sure the current climate models cannot represent what is actually happening in the atmosphere (Singer, p.140)

No one would disagree that CO2 is a greenhouse gas. Like other molecules consisting of more than two atoms, it absorbs infrared radiation and therefore prevents heat from radiating from the Earth’s surface. All other things equal, if CO2 were the main or even an important determinant of surface temperature, an increase in its concentration in the atmosphere would be of concern. But, all other things are not equal. The impact of CO2 concentration, or more pertinently, the impact of man-caused CO2 emissions, is completely swamped by changes in phenomena that are entirely naturally occurring.

Sadly, the SPMs do not accurately and fairly reflect the findings from a plethora of scientific papers underlying the IPCC ARs. Rather, they have been highly edited  to support the agenda of climate activists.  For example, the AR5 SPM statement quoted above is entirely misleading. Though you would never know this from perusing mainstream media reports, the AR5 backs off from various alarmist claims. Here are some examples: from the IPCC’s SPM (quotes in italics):

• The rate of warming over the past 15 years (1998-2012; 0.05^oC per decade), which begins with a strong El Nino, is smaller than the rate calculated since 1951 (1951-2012; 0.12^o C per decade). Translation: the pace of global warming was both miniscule and decelerating over this period, despite the fact that CO2 concentration over the same period rose 26% from 312 parts per million (ppm) to 392 ppm.
•   Continental scale surface temperature reconstructions show, with high confidence, multi-decadal periods during the Medieval Climate Anomaly (year 950-1250) that were in some regions as warm as in the late twentieth century.” Translation: the current very slightly warmer period is NOT unprecedented. Since atmospheric concentration of CO2 was only about 200 ppm during what scientists call the Medieval Warming Period (MWP), it follows that variations in CO2 concentration have little or nothing to do with variations in the temperature of the Earth’s surface. Alarmists conveniently overlook this inconvenient fact.
• It is very likely that the annual mean Antarctic sea ice extent increased at a rate of 1.2 to 1.8 percent per decade… between 1979 and 2012. This rate is way below the projections of the IPCC’s global climate models (GCMs).
•  Equilibrium climate sensitivity is likely in the range 1.5^oC to 4.5^oC…and no best estimate for equilibrium climate sensitivity (ECS) can now be given because of lack of agreement on values across assessed lines of evidence. ECS is the amount of warming expected to occur with a doubling of atmospheric CO2. Translation: the sensitivity of the climate to CO2 in the atmosphere remains highly uncertain.

Even though these and other “walk-backs” in the IPCC report disconfirm alarmist rhetoric, none were reported in the media. There is no escaping the conclusion that the IPCC’s SPM grotesquely distorted the truth.

The mainstream media as usual seem more interested in promoting the official narrative than in telling the truth. The public is not being told elementary truths: for example,  global temperature records are unreliable, the changes … reported are trivial compared to natural variation… (Singer, p.182).