Have philosophers neglected human obligations to other animals ?
No, not all of them. Which is unsurprising given how many philosophers there have already been and how important animals are to human concerns.
Some notable ones that come to mind:
Jeremy Bentham, back in the late 18th century, somewhat "famously" within the sphere of animal ethics, wrote:
The day has been, I grieve to say in many places it is not yet past, in which the greater part of the species, under the denomination of slaves, have been treated by the law exactly upon the same footing as, in England for example, the inferior races of animals are still. The day may come when the rest of the animal creation may acquire those rights which never could have been withholden from them but by the hand of tyranny. The French have already discovered that the blackness of the skin is no reason why a human being should be abandoned without redress to the caprice of a tormentor. It may one day come to be recognized that the number of legs, the villosity of the skin, or the termination of the os sacrum are reasons equally insufficient for abandoning a sensitive being to the same fate. What else is it that should trace the insuperable line? Is it the faculty of reason, or perhaps the faculty of discourse? But a full-grown horse or dog is beyond comparison a more rational, as well as a more conversable animal, than an infant of a day or a week or even a month, old. But suppose they were otherwise, what would it avail? The question is not Can they reason?, nor Can they talk?, but Can they suffer?
Peter Singer, a well known Australian philosopher of ethics and professor at Princeton, wrote Animal Liberation in 1975 and has been a champion of extending ethics robustly to animals, including by adopting a vegan diet.
Tom Regan, who died last year in 2017, was an American philosopher who specialized in animal rights theory.
Rosalind Hursthouse, a British-born New Zealand moral philosopher noted for her work on virtue ethics and who wrote "Applying Virtue Ethics to Our Treatment of the Other Animals".
Gary Francione, currently a professor of law at Rutgers, who approaches these issues informed by his background in law and legal theory, but whose points probably should fall under the category of ethics (and therefore philosophy) is another advocate of animal rights.
Robert Garner, approaching this from his position as a professor of political theory, wrote A Theory of Justice for Animals in 2013.
And, of course, there are many more. Other respondents here may offer up some additional great examples.