OIS
OIL STATES INTERNATIONAL, INC.
stock NYSE

IME I think it has a lot to do with the various complexities that come along with fixed income markets in general.

The most obvious one is the absolute archaic plethora of conventions that most fixed income markets use, that are "standard" for those markets. I've hand-built yield curve generators before (which tie out to Bloomberg's results to 10+ decimal places), and the whole process of transforming and normalizing JUST the day count conventions is a complete nightmare. Beyond that, you have to bootstrap the entire curve based on multiple instruments (OIS, T-bills, STIR futures - which were Eurodollar futures before the SOFR transition - T-notes, T-bonds, swaps, etc.), and each different curve type requires it's own set of underlying indices, which each have their own set of underlying securities that the constant-maturity point is derived from. Then you may or may not have to apply OIS discounting to the whole thing, it depends on the use case of the curve you're generating. Long story short, it's fucking complicated just to get a full calibrated curve for your particular use case.
Second is the data availability. You have to understand that the fixed income market is orders of magnitude larger than the equity market, both in terms of total size and number of instruments traded. With equity, there's typically one common stock class you have to worry about. Yes, there's preferreds, secondaries, warrants, and all that jazz. But each fixed income security has limited issuances, and essentially their own micro-markets. Sourcing the most up-to-date and real-time data is a MASSIVE undertaking, and honestly no firm really has the capital to constantly be subscribed to every data feed in the bond market all at once (nor would you really want to TBF). In the simplest terms, you only need 1 feed for common stock, but a feed for all the corps on that same ticker could equate to tens or hundreds of CUSIPS on the same company.
Third is just the sheer mathematical rigor you need to fully understand fixed income securities, especially when it comes to portfolio management. I guarantee you the average college grad can't tell me the difference between effective duration and Macaulay duration, much less when the appropriate time to use each would be. Add on to that all of the different methodologies for measuring convexity, credit risk, spread calibration, etc. Any sort of person you trust to work with that type of stuff has to be cultivated and specialized, to a level where they can intuit about things, not just get the math right.
Hopefully that gives you a glimpse - I'm sure there's plenty I'm missing that others can chime in on. But yeah. Bonds are fucking hard.

IME I think it has a lot to do with the various complexities that come along with fixed income markets in general.

The most obvious one is the absolute archaic plethora of conventions that most fixed income markets use, that are "standard" for those markets. I've hand-built yield curve generators before (which tie out to Bloomberg's results to 10+ decimal places), and the whole process of transforming and normalizing JUST the day count conventions is a complete nightmare. Beyond that, you have to bootstrap the entire curve based on multiple instruments (OIS, T-bills, STIR futures - which were Eurodollar futures before the SOFR transition - T-notes, T-bonds, swaps, etc.), and each different curve type requires it's own set of underlying indices, which each have their own set of underlying securities that the constant-maturity point is derived from. Then you may or may not have to apply OIS discounting to the whole thing, it depends on the use case of the curve you're generating. Long story short, it's fucking complicated just to get a full calibrated curve for your particular use case.
Second is the data availability. You have to understand that the fixed income market is orders of magnitude larger than the equity market, both in terms of total size and number of instruments traded. With equity, there's typically one common stock class you have to worry about. Yes, there's preferreds, secondaries, warrants, and all that jazz. But each fixed income security has limited issuances, and essentially their own micro-markets. Sourcing the most up-to-date and real-time data is a MASSIVE undertaking, and honestly no firm really has the capital to constantly be subscribed to every data feed in the bond market all at once (nor would you really want to TBF). In the simplest terms, you only need 1 feed for common stock, but a feed for all the corps on that same ticker could equate to tens or hundreds of CUSIPS on the same company.
Third is just the sheer mathematical rigor you need to fully understand fixed income securities, especially when it comes to portfolio management. I guarantee you the average college grad can't tell me the difference between effective duration and Macaulay duration, much less when the appropriate time to use each would be. Add on to that all of the different methodologies for measuring convexity, credit risk, spread calibration, etc. Any sort of person you trust to work with that type of stuff has to be cultivated and specialized, to a level where they can intuit about things, not just get the math right.
Hopefully that gives you a glimpse - I'm sure there's plenty I'm missing that others can chime in on. But yeah. Bonds are fucking hard.