Frontend code sees a lot more refactoring and tweaks than backend code, this is mostly because frontend is visual and the client's demands are fluid, a client will stumble on something on another website and request for that same feature thus promoting changes in the code base, also information displayed on a website or platform is frequently updated (depending on the organization).
Clients however perceive backend code as sacred, if it already works they fear a small change might bring down the house of cards so they avoid this except its absolutely necessary or something has gone horribly wrong. As a result of this I believe backend engineers are more likely to work under pressure than frontend engineers.
In a way, backend engineers will always have to spend free time rapidly improving their skills because unlike frontend where one can present something visually close to what is expected, backend is more of an exact science in that if there's a compiler error or runtime error the code is virtually useless until it's somewhat perfect. The needs of clients are rapidly evolving as they innovate to outperform the competition, we are their ace in the hole.