plus, to regularly spend bitcoin one also has to find a way to regularly earn it
otherwise buying bitcoin for spending it is too costly and it makes no sense
What you say makes sense, looking at Bitcoin in a linear way.
Buying, waiting for it to appreciate, and then spending isn't exactly expensive. Rather, it's a way to earn and spend without losing. Now, we know this isn't a given either. But for those who have been in Bitcoin for a long time and have held on, it now makes sense to spend.
Therefore, in some cases, it may make sense to buy and then spend. Not with the idea of buying and spending in the immediate term, but in the medium or long term.