Because with the stock 1x/10x probe the oscilloscope can reach vertical resolution only 100 V/div.
The scope has a well-defined input impedance of 1 megaohm. A 1x probe is connected directly, a 10x probe has a 9 MΩ resistor in series and acts as a 1:10 voltage divider, with 10 MΩ impedance.
A further series resistance can increase the division ratio. Attaching a 90 MΩ in series results in a 1:10 divider with the 10-megaohm probe, or 1:100 divider relative to the scope input. Voila, a 100x probe.
A stock of metallized 10-megaohm, 1% resistors was obtained. The resistors are composed of a ceramic body with the resistive layer deposited on top, and with copper caps on its ends that act as the contact terminals. The caps are crimped to the ceramic body, with the joint having decent mechanical integrity.
The leads were cut off right next to the resistor body, and the protective paint coating from the caps was removed using a small milling bit. The exposed cap end was then solder-coated. Nine resistors then were soldered together, butt-to-butt, forming a rigid resistor bar.
A header pin was soldered to one end, acting as a probe tip. A small lug was formed on the other end of the bar.
A silicone tube was pulled over the bar, acting as a first level of insulation. The tube was secured against sliding with a piece of hot-melt-adhesive lined heat shrink tubing, which then were overlayed with another piece of the same tubing. A thicker silicone tube then was placed around this assembly as a second level of insulation, and secured with another piece of a heat-shrink tube. This tube was somewhat longer, reaching over the lug.
The 10x probe is converted to 100x probe by attaching to the lug. The level-2 insulation provides electrical insulation of the probe-lug connection and adds some further mechanical integrity.
The probe was tested up to 2100 volts and was found to work well. The voltage match between the 10x and 100x probe on the same voltage input was found to be within 1%.
90 MΩ resistor bar
Level 1 insulation
Level 2 insulation
Attached probe, detail