I use mine like everyday and the combined time is like 8 hours on end! I just wanna know why they last for so long.

They very rarely get hot, just because they are heavy and on for only few minutes with way longer for cooling.
Plus even when the microwave is 10+ years old, the actual runtime is barely few 100's hours. If you are using it in some business activity really heavily, still it reaches few k-hours. At that time many other parts on the microwave fail way sooner and with many from these it is not economical to replace or even their failure makes the thing unsafe even after repairs (rusted case, broken magnetron sputtering berylium ceramics around, broken magnets,...)

Compare that with lighting use, where 10 years in street lighting means 36k hours and majority of that close to the top temperature rating, yet even there the prime failure/replacemects cause uses to be corrosion and system becoming outdated and not the ballast failing alone...

Added to that is the fact that a transformer is a static device, with no moving parts. Provided it's in an environment where it isn't getting too hot and has adequate ventilation and/or cooling, there's not a lot to go wrong. Ballasts in street lamps have been known to last 40 years and up.

There have been Microwave transformer failures from a shorted rectifier diode,cap or magnetron.They can fail.Early magnetron tubes had glass envelopes rather than ceramic.If the ceramic is intact-no hazard.Now if its broken from someone trying to salvage the magnet than its a hazard.You don't want to get cut by the fragments or breathe its dust.Best to leave the ceramic tubes intact.Tube collectors may get a kick out of them.Transmitter tubes have the same ceramic case.so we just replace them and let the rebuilder deal with it.The large transmitter tubes can be rebuilt-the envelope opened-bad parts-filament,and other replaced the tube reassembled,repumpted and tested for reuse.

There have been Microwave transformer failures from a shorted rectifier diode,cap or magnetron.They can fail.

What I wanted to say it is usually not the transformer to fail first, but very often the transformer fails as victim of the other parts failure.
Most frequently the capacitor fails as first, which then fries the diode and the HV fuse (in case of a short circuit failure). The secondary open circuit means there is no capacitive reactance load, so the primary gets overloaded by the inductive reactance current imposed by the core design (normally designed to cancel out the capacitive load nature).
Other sequence frequently occuring is the capacitor loosing capacitance, leaving extra inductive reactance on the primary so overloading it, plus the reduced magnetron power (because of the lower current feed) makes the user to operate the thing longer.
Rarely the magnetron goes away, but I've seen just one occassion and even that was more likely caused by overheating magnets loosing field because of mess accumulation on the cooling fins and on the fan.
These then end up very often by fried MOT, but can not be counted as MOT caused failures.

I heard microwave popcorn is bad for microwaves, because there isn't enough matter to heat, so the magnetron overheats.

I heard microwave popcorn is bad for microwaves, because there isn't enough matter to heat, so the magnetron overheats.

If you do that in the bags designed for microwaving it (the "microwave popcorn" is sold in thse), it is OK. These bags have build in resistive metallic layer designed to absorb the microwaves and turn them into heat. Yes, it is then just using a disposable pan heated by a lossy magnetron, but for the magnetron it is OK.
Worse would be if someone attempts to make the popcorn without those bags, that will overheat the magnetron. But reasonable microwave designs should count for that and feature proper protection. But still it is a stress for the components involved...

We've had our current microwave for well over ten years and the only reason we will be replacing it later on in the year is next to our new  cooker it is looking past its best.

这在中国是不存在的，因为中国的微波炉变压器大部分都是铝制线圈的，很快就会烧毁。。

I still use an Amana RadaRange from the mid 70's. Has a couple dials rather than a touchpad and digital display. It is built like a tank compared to anything modern, it weighs about 80 pounds. The new ones have undersized transformers that still perform OK, but the old ones will run for hours without getting very hot.