They aren't everywhere. Some places are experiencing warmer winters. There remains an overall increase in temperature of the planet, both winter and summer.
Global warming is not evenly spread over the planet or over time.
However, there is no denying that in some places in the northern hemisphere, winters have been sometime colder and seem to last longer.
This is consistent with the global warming that is affecting all the planetary systems. The arctic is actually warming at about twice the rate of the rest of the earth. (Its still cold , just not as cold and getting warmer). This affects the jet stream which is the wavy river of air surrounding the arctic. This "waviness" and the speed with which the stream moves is what drives weather events in the northern hemisphere.
The warmer temperatures in the arctic are causing something called "Arctic Amplification." One result of that is that both the size of the waves of the stream is increased (it dips higher and lower than has been the case,) and its motion is slowed down.
This results in the cold temperatures from the arctic dipping further down toward the equator, and storms and weather that last longer, not moving on as quickly. There is some suggestion that as the arctic warms we will see, for a time, cooler temperatures in parts of Europe.
This phenomenon requires more investigation. It does point out how our life support systems and global cycles are being affected dramatically by what are still relatively small temperatures changes.
Further explanation of the Arctic Amplification can be found here: https://www.youtube.com/watch?v=5a5DJVcSh8A