They're trying; it's the subject of the second paper referenced at the end of the link above (direct link: <https://iopscience.iop.org/article/10.3847/2041-8213

They're trying; it's the subject of the second paper referenced at the end of the link above (direct link: <https://iopscience.iop.org/article/10.3847/2041-8213/ad2df1 https://iopscience.iop.org/article/10.3847/2041-8213/ad2df1>, open access). It's a bit complicated but (for our galaxy's central SMBH, Sgr A), from simulations they have a confidence interval spanning a few tens of Gauss at significant radial distance to a few hundred Gauss near the event horizon, or an average somewhere below 30 Gauss. Early measurements of M87 (<https://iopscience.iop.org/article/10.3847/2041-8213/abe4de https://iopscience.iop.org/article/10.3847/2041-8213/abe4de>) arrived at somewhere between one and thirty Gauss (on average) for that black hole. By comparison, Earth's magnetic field is about 0.5 Gauss at the surface. So Sgr A's averaged magnetic field is strong compared to that. However... The Earth's magnetic field strength increases with depth below the surface, reaching about 25 Gauss at the core, which is a lot less than near the horizon of Sgr A. But sunspots are on the order of a thousand Gauss. The strongest magnetic fields human technology has been able to generate is on the order of tens of millions of Gauss. A magnetar's field strength is at least tens of billions of Gauss. Our black hole has not been feeding recently (as in, for the past many thousands to millions of years dating from the view we have of it here and now), so doesn't have much matter around it to drive the field strength higher. M87* is not feeding much either. The best working model for magnetars describes an early non-equilibrum state for the magnetar in which at intermediate depth a complicated nuclear fluid acts as a proton-dominated superconductor. Flows within that fluid act as a dynamo (well, really a magnetohydrodynamic (MHD) generator) for the magnetic field. A significant accretion structure around a black hole can also act as as an MHD generator. Weakly feeding black holes lack such a structure.