Even if it's homogenous, you have steadily increasing stresses with depth. This occurs for perfectly uniform materials as well. If you can't offset these to keep the hole open, it collapses.
Rocks are very weak in tension, despite being very strong in contraction. It's the reason you can break rock with a hammer or the reason ancient quarries were able to work by pouring water on wood pegs in rocks. It's also the reason concrete needs rebar to reinforce it (steel is very strong in tension, so the two combined are exceptionally strong). Keeping a hole open requires strength in tension as well as strength in compression.
Drilling mud accomplishes this by being roughly the same density as the rock, so it offsets the stresses that are trying to close the borehole that steadily increase with depth due to the increasing amount of rock above. Drilling mud keeps the borehole open until you can put in casing to support it.
This is exactly the same reason why it's difficult to build a submarine that can go to very large depths in the ocean. To put up steel walls (casing) to keep it open, you have to stop drilling and cement in casing - you can't do that as you drill. So drilling mud is a key part of being able to drill efficiently. Otherwise, you'd need to stop every few tens of meters and spend _days_ setting casing before being able to drill again.
Regardless, there is nowhere on earth where things are homogenous over very long distances. Simply put, even relatively uniform rocks can have very significant variations in physical properties. Many relevant properties (e.g. permeability - how well fluids can move through) vary over _tens of orders of magnitude_ naturally. So "uniform" can still mean "only varies by a few orders of magnitude". There are places where you can reasonably avoid non-silicates, but you're going to hit tons of other issues due to fundamental heterogeneity.
Rocks are very weak in tension, despite being very strong in contraction. It's the reason you can break rock with a hammer or the reason ancient quarries were able to work by pouring water on wood pegs in rocks. It's also the reason concrete needs rebar to reinforce it (steel is very strong in tension, so the two combined are exceptionally strong). Keeping a hole open requires strength in tension as well as strength in compression.
Drilling mud accomplishes this by being roughly the same density as the rock, so it offsets the stresses that are trying to close the borehole that steadily increase with depth due to the increasing amount of rock above. Drilling mud keeps the borehole open until you can put in casing to support it.
This is exactly the same reason why it's difficult to build a submarine that can go to very large depths in the ocean. To put up steel walls (casing) to keep it open, you have to stop drilling and cement in casing - you can't do that as you drill. So drilling mud is a key part of being able to drill efficiently. Otherwise, you'd need to stop every few tens of meters and spend _days_ setting casing before being able to drill again.
Regardless, there is nowhere on earth where things are homogenous over very long distances. Simply put, even relatively uniform rocks can have very significant variations in physical properties. Many relevant properties (e.g. permeability - how well fluids can move through) vary over _tens of orders of magnitude_ naturally. So "uniform" can still mean "only varies by a few orders of magnitude". There are places where you can reasonably avoid non-silicates, but you're going to hit tons of other issues due to fundamental heterogeneity.