TLDR: Refine rates in the game are absolutely awful and to refine it is extremely expensive.
The current cost of legendary anvils (which are not bought by the overwhelmingly majority of players on the server) gives credence to the fact that the cost a player expends to refine a certain equipment to say, level 13, is much too high given the amount of effort required to accrue the resources to even do so in the first place.
From levels 10-13 onwards, you would need over 20 million to buy enough legendary anvils (adds 10% refine rate) to use for one try. Most players do not invest in this because of the high cost. One may argue the extra 10% is more beneficial then gambling with a 10% and 5% rate from lvls 9-12 and lvl 13 respectively, however RNG is ultimately RNG. For example, I have had times where I went from lvls 10-13 on my equipment in less then 10 tries, and times where I have not even been able to surpass lvl 9 after 30 tries. One could say I should have bought legendary anvils, but the cost is a barrier.
Still, the argument that one should invest in them anyway (as of now) does not take away from the fact that refine costs are absurd. The best solution would be to change the price of anvils (i.e. make a package of 10 instead of 5, and sell it for less then 10k myshop points). This would be a good solution as it is already frustrating to fail 10-30 times on one given level.
NOTE: Currently legendary anvils can only be used up until lvl 11 IIRC, if they can be extended to be used for lvl 12+13 that would also help. Alternatively, extending god anvil use to lvl 13 would also be another solution to consider (it has 25% increased rate).