A similar argument has been made for WSL referees to be full time, not so much by the officials but rather by clubs who have been on the wrong end of bad decisions. The simple fact is though, and I would guess it is the same in Spain, there just isn't the money to pay for it. The average attendance for the WSL last season was under 2,000, the average EPL attendance is close to 40,000. It may well have increased significantly due to the effect of the Euros, but still massively lower and the FA's goal is to get it to 6,000.
The WSL TV deal is worth £8 million per season, the EPL TV deal is worth more than £3 billion per season, then we have the vast difference between player salaries. I'm going back a few years, but WSL referees got £120 per game, EPL referees got over £1000 per game on top of their annual salary. I know the strike is Spain related and not the WSL, but the same principle surely applies, namely where do they expect the money to have parity is coming from?