1 New phones are very weak
The fact that many new phones are very weak compared to the old Nokia phones made many people make comparisons between the new and old phones.
2 Spreading of jokes
Many jokes spread about the issue. Many people started to believe that Nokia phones are indestructible as a result of those jokes.
3 Some Nokia phones are really tough
Many of Nokia's phones are tough and highly durable. Some of those phones can hardly get affected no matter how hard it falls on the ground.
4 Nokia phones are rigorously tested for durability
According to an ex-Nokia employee, Nokia phones are rigorously tested for durability. Many tests are applied on the phones such as drop tests, bending tests and temperature related tests.
5 It matches the Finnish culture
According to a Quora thread, Finnish people see themselves as rugged and strong. They build strong products and take pride when those products prove their strength.
6 They last for long periods of time
Many Nokia phones last for long periods of time even with continuous use. This has led people to believe that the phones are indestructible.
7 Some Nokia phone are waterproof
Many Nokia phones are water resistant, while some are completely water proof. There are many incidents of Nokia phones being brought back to life after falling in water. (See Is Apple Watch Series 4 Waterproof?)
8 Stories about durability
People spread many stories about Nokia's durability online. Some people reported that their Nokia phones survived a drop from a 5 story building, while others said that their phone survived after being run over by a vehicle. (See Why do iPhones break so easily?)
9 New phones use different material
At the time Nokia dominated the mobile phone industry, relatively heavy material was used, which made the phones stronger. As phones used lighter materials, they became much weaker.