Yes agree they dont crack these days , but during 17th 18th century they did , remember tea also came from india , it was during the british colonial rule of india that british people were introduced to tea drinking and china cups were often of poorer quality

Taken from an article on the net :-

It's not about the taste.
With the weather turning cooler, most people turn to hot cocoa, PSLs and other cozy drinks. Personally, though, I prefer a piping hot cup of tea—whether it’s green, herbal or black, I can’t think of a better way to wake up for the day. Plus, it has tons of health benefits.

The one thing I can’t get behind though, is adding milk to my tea—despite my family’s love of chai. However, it got me thinking—why put milk in tea, anyway? Turns out, it’s a very British thing to do. And it has nothing to with the taste.

Why Put Milk in Tea?
The Brits’ habit of putting milk in tea extends all the way back to the 18th century, from the time when tea was brewed in pots. Tea was a big deal at the time, and people tended to drink it out of china cups. However, most people couldn’t afford fancy fine bone china, and the cups available would crack from the heat of the boiling hot tea.

The solution? Pour milk into the cup first, then add the tea. The cold milk cooled down the tea enough to keep the china from breaking, and well, the reduced bitterness was just an added benefit! According to some sources, tea was also incredibly valuable at the time, so families who couldn’t afford large amounts would add a large amount of milk and a splash of tea, while well-off families tended to do the opposite.
This is Queen Elizabeth’s favorite tea—and where to find it.

Is There a Right Way to Drink Your Tea with Milk?
Not really. You can add as little or as much as you please—though adding milk first and then pouring the tea on top might make it taste better. This is because when you add hot tea to cold milk, you’re bringing the milk to the temperature of the tea, distributing the flavor more evenly. However, if you add milk to tea, you’re cooling the tea down, and it may not taste as good.

And if you want to make your tea a little prettier, grab one of these tea bombs!